Strip relative paths from android_library_import output files

androidx.annotation_annotation is used as a test data file, and
converting it from an android_library to an android_library_import
causes the relative path used in the test data path to change.
Clear the relative path in android_library_import the same way that
android_library and other java based modules do.

Also change the name of classes-combined.jar to the name of the
module so that the output file has the right name if there are no
static dependences that require running the merge_zips step.

Bug: 288358614
Test: TestAndroidLibraryOutputFileRel
Change-Id: I28210aa370a742d789102ff71db3685ca744878f