Add a flag-protected (-pom2build) feature for pom2bp to produce Bazel BUILD files.

Note that this is an initial, rather crude attempt at this,
that unblocks generating BUILD files needed to build AOSP android apps.
In particular, note that
- Jetifier step currently ignored
- Visibility set to public for all output targets
- All dependencies are gathered in 'deps', though they are split under various categories in bp files
- Host, host-only notions are ignored for now

Test: Copy first 2 lines of, e.g. prebuilts/sdk/current/support/Android.bp to a BUILD.bazel file and run pom2bp -pom2build -regen BUILD.bazel
Bug: 198433754
Change-Id: I5efa93517e6c319fd828da5b16ba7efd2d90bca8