add gn_android_flavor

add gn_android_flavor

This ought to support compiles for now.

Am I picking up my CIPD ndk packages right?

The main thing to note is that I'm passing the target_arch directly through
as target_cpu.  This means these bots will have a slightly different naming
convention than we've been using, but it'll agree with what you must type
yourself when using GN to build for Android:

  - Arm7 -> arm
  - Arm64 -> arm64
  - Mips -> mipsel
  - Mips64 -> mips64el
  - x86 -> x86 (unchanged)
  - x86_64 -> x64

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2292663002

Review-Url: https://codereview.chromium.org/2292663002