Enable density-based APKs in Bazel

[BenHenning]

This is derived from #2259. Bazel should have multiple builds with the following configurations:

• Density-based production builds with proguard/appreduce enabled
• Developer build with all density files included and no proguard/appreduce

This functionality will be new in Bazel since support for multi-apk builds needs to be turned off in Gradle.

[BenHenning]

We now have a very robust set of builds (with dev-specific features) for Bazel:

oppia-android/build_flavors.bzl

Line 45 in 0ef7a46

_FLAVOR_METADATA = {

There really isn't much advantage in going further than this since we are only uploading AABs now, and are probably going to remove APKs entirely in the future (since AABs allow Play Store and bundletool to generate density-specific APKs at installation time rather than requiring the Oppia team to manage multiple density-specific APKs).

With that in mind, I don't actually think we have any work to do here.