bp2build: fix exclude_srcs in subpackages.

In a non-top level Android.bp file, exclude_srcs was not working at all
due to a bug in expandSrcsForBazel. GlobFiles was expanding a glob
relative to root, but the expandedExcludes list was relative to the
module dir, causing the glob function to not consider the
expandedExcludes list at all.

Add tests to demonstrate that this is working now.

Test: TH
Bug: 186388919
Change-Id: Ice8254231d085b39126e91b823a09ec328ee0ae0