logo: Use new design, and support fancy Android features

Fixes #415.
Fixes #1254.
Fixes #1401.
Fixes #1402.

Thanks to Vlad for developing the new design through several
iterations.  Changes from the old version include:

  * The background gradient is more saturated.  (#1254)

  * The "BETA" label is cleaner, and has a more balanced layout
    relative to the main "Z" of the logo.  (#415)

  * The icon is "adaptive" using the API introduced in Android 8,
    which lets it be handled more nicely by Android launchers /
    home screens that want to put the app icon in a circle (like
    on Pixel) or an iOS-style squircle (like on Samsung devices).
    In particular this should avoid it getting shrunk and placed on
    a bed of white padding.  (#1402, originally filed on zulip-mobile
    in 2020.)

  * The assets include a monochrome foreground icon, which makes
    the app work with the "themed icons" introduced in Android 13,
    for users who choose to enable that.  (#1401, originally filed
    on zulip-mobile by a user report last year.)

This commit continues to use "BETA" versions of the logo for the
actual app, but also adds the corresponding non-beta versions of
the source SVGs so that we can smoothly switch to those when it
comes time to launch the app.

The relevant area in Figma is around here:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6549-5962&t=mMvSGpqwbgJBIv6U-0

but the exact story of these source files is somewhat complicated,
because there are a number of technical requirements and it's taken
several rounds of iteration between design and engineering to get to
this result.

Specifically:

  * zulip-white-z-beta-on-transparent.svg was "beta-foreground.svg"
    from Vlad here:
      https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/updated.20app.20icon/near/2072480

  * zulip-white-z-on-transparent.svg and zulip-gradient.svg were
    from Vlad in this message (as code blocks):
      https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/updated.20app.20icon/near/2038233

  * Then zulip-combined.svg was "app_icon_1.svg" here:
      https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/updated.20app.20icon/near/2030417

    which I produced as described in that message from "app_icon.svg"
    here:
      https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/updated.20app.20icon/near/2030569

  * And zulip-beta-combined.svg I assembled myself, in a text
    editor, out of those pieces.

  * I also wrote ic_launcher.xml.

Author: gnprice

android/app/src/main/res/mipmap-anydpi/ic_launcher.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+  <background android:drawable="@mipmap/ic_launcher_background"/>
+  <foreground android:drawable="@mipmap/ic_launcher_monochrome"/>
+  <monochrome android:drawable="@mipmap/ic_launcher_monochrome"/>
+</adaptive-icon>

android/app/src/main/res/mipmap-hdpi/ic_launcher.webp

@@ -45,23 +45,18 @@ jq --version >/dev/null 2>&1 \
     || die "Need jq -- try 'apt install jq'."
 
 
+# White Z above "BETA", on transparent background.
+# TODO(#715): use the non-beta version
+src_icon_foreground="${root_dir}"/assets/app-icons/zulip-white-z-beta-on-transparent.svg
 
-# Suppose (for comments below) this points to a zulip.git worktree.
-# zulip_root="${root_dir%/*}"/zulip
+# Gradient-colored square, full-bleed.
+src_icon_background="${root_dir}"/assets/app-icons/zulip-gradient.svg
 
-# White Z in gradient-colored circle.
-# src_icon_circle="${zulip_root}"/static/images/logo/zulip-icon-circle.svg
-
-# White Z in gradient-colored circle with BETA banner.
-# Contains a near-verbatim copy of ${src_icon_circle}.
-src_icon_circle_beta="${root_dir}"/assets/app-icons/zulip-icon-circle-beta.svg
-
-# White Z in gradient-colored square, full-bleed.
-# src_icon_square="${zulip_root}"/static/images/logo/zulip-icon-square.svg
-
-# White Z in gradient-colored square, full-bleed, with BETA banner.
-# Contains a near-verbatim copy of ${src_icon_square}.
-src_icon_square_beta="${root_dir}"/assets/app-icons/zulip-icon-square-beta.svg
+# Combination of ${src_icon_foreground} upon ${src_icon_background}...
+# more or less.  (The foreground layer is larger, with less padding,
+# and the SVG is different in random other ways.)
+# TODO(#715): use the non-beta version
+src_icon_combined="${root_dir}"/assets/app-icons/zulip-beta-combined.svg
 
 
 make_one_ios_app_icon() {
@@ -72,7 +67,7 @@ make_one_ios_app_icon() {
     local output_basename=Icon-"${size_pt}x${size_pt}@${scale}x".png
     local output="${iconset}"/"${output_basename}"
     if [ ! -f "${output}" ]; then
-        rsvg-convert "${src_icon_square_beta}" -w "${size_px}" -o "${output}"
+        rsvg-convert "${src_icon_combined}" -w "${size_px}" -o "${output}"
     fi
 
     printf >>"${contents}" \
@@ -152,9 +147,10 @@ make_android_icon() {
 }
 
 make_android() {
-    # Launcher icon goes in a mipmap:
+    # Launcher icons go into mipmaps:
     #   https://developer.android.com/training/multiscreen/screendensities#mipmap
-    make_android_icon "${src_icon_circle_beta}" 48 main mipmap ic_launcher
+    make_android_icon "${src_icon_background}" 108 main mipmap ic_launcher_background
+    make_android_icon "${src_icon_foreground}" 48 main mipmap ic_launcher_monochrome
 }
 
 make_ios