Output of an apex module type can be included in tests

By implementing SourceFileProducer interface, output of an apex module
can be included in tests (via ':modulename' syntax in 'data' property)

Bug: 120055902
Test: replace apexd_testdata/* with :apex.test in
system/apex/apexd/Android.bp. m apex_file_test.
out/host/linux-x86/nativetest/apex_file_test/ has apex.test.apex in it.

Change-Id: I8b721a68e0edc65a3e674febeed485e06b96bc24