gh-146445: Migrate Android build tools to the Platforms folder.

[freakboy3742]

Moves the Android build folder into the Platforms.

For consistency with iOS and Emscripten, the entry point is now named Platforms/Android (rather than android.py), but the android.py name is preserved in the release artefact.

This should not be landed until python/buildmaster-config#699 lands.

Fixes #146445.

• Issue: Move Android folder to Platforms #146445

[freakboy3742]

!buildbot android

[freakboy3742]

The buildbot failure is relatively straightforward fix; but in the process of debugging the buildbot failure, I discovered that custom cross-build directories aren't being propagated to Gradle; see #148319 for the fix; that should be merged before this PR.

[freakboy3742]

!buildbot android

[mhsmith]

I've added a "Testing Python" subsection to the README to separate the Python test suite instructions from the content that's shared with third-party packages.

[freakboy3742]

!buildbot android

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @freakboy3742 for commit 170a4ae 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F148282%2Fmerge

The command will test the builders whose names match following regular expression: android

The builders matched are:

• AMD64 Android PR
• aarch64 Android PR