Add CI Android targets

[DellaBitta]

Adds a CI Workflow to build the testapps against the publicly available packaged SDK.

• Borrows heavily from the Firebase C++ SDK build scripts.
• Nightly builds at 1am PST.
• Builds triggered on PRs.
• Some updates to the android build dependencies in order for the build to succeed with the cmake version installed on the GitHub CI runners.
• Add build verification using GitHub Actions #23

[DellaBitta]

Added this file to this Android-only build PR because build_testapps.py refers to it and I thought it would be better to submit it now and alter it later than to prune build_testapps.py of all xcode references only to add them back in later.

It seems safer this way.

[DellaBitta]

File is mainly a directly copy of the one from the C++ SDK repo. Mainly changes have been made to the paths.

[DellaBitta]

Nearly a direct copy of the C++ SDK version, with the removal of all of the vcpkg functions.

[DellaBitta]

Nearly a direct copy of the Firebase C++ SDK version, but the comments which contain the example have been updated with the test app bundle ids / naming convention.

[sunmou99]

LGTM.
Because the build scripts are almost identical to the scripts in C++ repo, it maybe better to create a github action which handles environment setting steps and the build steps in the future.

[DellaBitta]

Yes, I was thinking along the same lines!