Default from-text stub generation in build

This change defaults Java stubs to be generated from API text files
during build. Using the `--build-from-source-stubs` flag, users can
toggle between the feature.

Test: m nothing && verify ninja path exists between android_stubs_current and android_stubs_current.from-text, and does not exist between android_stubs_current.from-source, m nothing --build-from-source-stub && verify the opposite
Bug: 274805756
Change-Id: I28834f92c1b1311e3fe0a71a6ea9e8ec2e278d7e