Surface Java APIs Used By APK-only Modules.

Previously we were only generating used-by API-coverage for APEX modules.
This change adds support for APK-only modules such as NetworkStack and DocumentsUI.

Bug: b/216313756
Forrest Run: https://android-build.googleplex.com/builds/abtd/run/L10800000953846781

Test: TARGET_BUILD_VARIANT=userdebug PRODUCT=mainline_modules_x86 ./vendor/google/build/build_unbundled_coverage_mainline_module.sh -j16

Change-Id: Id17e4a55c2a52e9903632a654e778f8d54982dfc