Running a script like external/conscrypt/srcgen/generate_android_src.sh
removed the TEST_MAPPING files. This change restores them by checking
them out of git. This does mean that if a TEST_MAPPING file has been
edited that the changes will be lost.

Test: external/conscrypt/srcgen/generate_android_src.sh
Change-Id: Ifedff3823f14a7a098a677709bf1ce89b5a8c22d