Better partitioning module/non-module libraries

Instead of including source files in both libstagefright
and libstagefrightt_mpeg2extractor, have libstagefright include
the libstagefright_mpeg2extractor library. This preps us for
dis-entangling the source trees for those two.

Bug: 245354160
Test: build, boot
Test: CtsMediaExtractorTestCases
Change-Id: Iba0330c9ee257ddb8a5d41963ff0cc9dd8ecb64a