MediaAppender:File fmt mime to determine extractor

Instead of extractor name check, obtain mime from extracted file format.
Use this mime info to limit mediaappend to only .mp4 files.

Bug: 193487901

Test: atest android.mediav2.cts.MuxerTest

Change-Id: I40c3caad74b1e595525611951d005da413f4eadb
1 file changed