Generate .srcjar for prebuilt_stubs_sources

Changes prebuilt_stubs_sources to generate a .srcjar from its input
instead of just exposing the srcs it is given. This ensures that it can
be used as a drop in replacement for a droidstubs module.

Updates the test for prebuilt_stubs_sources to be more representative
of the actual use made of it by sdk snapshot which outputs a directory
not a glob pattern. Added some documentation of the
prebuilts_stubs_sources srcs property to make it clear that it is
supposed to be a set of directories.

Extracts common code from sdk/testing.go for normalizing path/paths
for testing.

Bug: 143678475
Test: m conscrypt-module-sdk conscrypt-module-host-sdk conscrypt-module-test-sdk
      unzip those in place of external/conscrypt
	  build core-current-stubs-source which expects it to provide a .srcjar.
Change-Id: I8204a022557a9b0b45e19eac79ecba98ff95213d