Fix inconsistent MIME type mapping.

MediaProvider heavily relies on developers to provide a MIME type,
which it then translates into a file extension, and then later back
into a MIME type.

For this flow to work without apps losing access to the data they
just wrote, all MIME types need to consistently map back to the
same "major" type that they started with.

This change adds tests to verify this consistency for all audio,
video, and image MIME types, and fixes an obscure bug where the
"audio/3gpp" MIME type would end up translating to "video/3gpp".

Bug: 154667531
Test: atest CtsMimeMapTestCases
Change-Id: I47998d8f4b1f9922a7d9439014e2f7f51f401f04