Delete aapt2 generated java files after creating srcjars

The R.java files generated by aapt2 are unused after they are zipped
into srcjars.  These files can huge, reaching 500 MB for some modules
and 76 GB across all modules for a checkbuild on internal main.
Delete the R.java files after after zipping them into srcjars.

Bug: 294285775
Test: m checkbuild
Change-Id: Ie5143e1e4b36ca1f4e45b001d79aca5379063517