Use filegroups for test-{base,mock,runner} srcs

This allows soong to infer the package of the given srcs, which in turns
make it able to filter the inputs to metalava to exclude unwanted
packages.

This is part of a larger topic of changes intended to remove undesired
classes from the public documentation.

Bug: 187386774
Test: m
Change-Id: I8196c306b743ce6ca8a6452d212f3d7c0b5b2b82