[et] Add Android x64 configurations

[dcharkes]

Closes: flutter/flutter#156598

[dcharkes]

android_debug_unopt_x64 works as expected.

android_debug fails for me locally though, but I can't spot what is wrong:

$ dart /usr/local/google/home/dacoharkes/flt/engine/src/flutter/tools/engine_tool/bin/et.dart build --rbe --gn-args=--no-prebuilt-dart-sdk --gn-args=--full-dart-sdk -c android_debug
[...]
Command failed: ./clang_x64/gen_snapshot --snapshot_kind=core --enable_mirrors=false --vm_snapshot_data=/usr/local/google/home/dacoharkes/flt/engine/src/out/android_debug/gen/flutter/lib/snapshot/vm_isolate_snapshot.bin --vm_snapshot_instructions=/usr/local/google/home/dacoharkes/flt/engine/src/out/android_debug/gen/flutter/lib/snapshot/vm_snapshot_instructions.bin --isolate_snapshot_data=/usr/local/google/home/dacoharkes/flt/engine/src/out/android_debug/gen/flutter/lib/snapshot/isolate_snapshot.bin --isolate_snapshot_instructions=/usr/local/google/home/dacoharkes/flt/engine/src/out/android_debug/gen/flutter/lib/snapshot/isolate_snapshot_instructions.bin /usr/local/google/home/dacoharkes/flt/engine/src/out/android_debug/flutter_patched_sdk/platform_strong.dill
exitCode: 254
Can't load Kernel binary: Invalid SDK hash.

[matanlurey]

android_debug fails for me locally though, but I can't spot what is wrong:

Looks like it fails on CI as well: https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/2912804/overview.

[dcharkes]

android_debug fails for me locally though, but I can't spot what is wrong:

Looks like it fails on CI as well: https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/2912804/overview.

Okay, I've removed the config. I only need one to test x64 Android emulators on Linux.

This is ready for review.

[matanlurey]

Thanks for the contribution!