[build] Download Android SDKs only on x64 platforms

This agrees with the Flutter engine DEPS, where the SDKs are not
downloaded on arm64 hosts because they are not available as CIPD
packages for arm64.

Bug: flutter/flutter#92911
Change-Id: I1a64c85fdac918734b60cf23a7c5034eba2a133b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/218922
Reviewed-by: Daco Harkes <[email protected]>

Author: whesse

DEPS

@@ -53,7 +53,8 @@ vars = {
   "checkout_benchmarks_internal": False,
 
   # Checkout Android dependencies only on Mac and Linux.
-  "download_android_deps": 'host_os == "mac" or host_os == "linux"',
+  "download_android_deps":
+    "(host_os == mac or host_os == linux) and host_cpu == x64",
 
   # Checkout extra javascript engines for testing or benchmarking.
   # d8, the V8 shell, is always checked out.