Allow java_test.go tests to use any Android.bp file

Previously, the tests had to use the Android.bp file in the root
directory of the mock file system. That prevented adding tests that are
dependent on the location of the Android.bp file, e.g. ones that use
no_standard_libs. This change will process any Android.bp in the mock
filesystem.

Bug: 134566750
Test: m
Change-Id: I6fb057a473a18e87bd1a89507e78ceb3fd171eb5