Refactored libstagefright_writer_fuzzer

The following are the updates to the fuzzer:
1. Added additional metadata.
2. Code Coverage improved by 40.06%.

Test: ./libstagefright_writer_fuzzer
exec/s: 910
Bug: 331540179

Change-Id: I8bc45b9e7c9fb611097a6be4f18a9a6cf0566e82