Add initial seamless transcoding tests

Added two new basic tests to verify
1) We return FileDescriptor of transcoded file for legacy apps
2) Multiple open() from same app doesn't result in multiple transcoding
requests

Bug: 169546642
Test: atest MediaProviderTranscodeTests
Change-Id: If241bb0286c872045fd816ce18b4d045d10e2b42
6 files changed