MPEG4Writer:Fix MetaData use-after-free issue

MediaCodecSource's mMeta would be replaced if encoder posts
OUTPUT_FORMAT_CHANGED message.  MPEG4Writer needs to hold
the old mMeta to use its variables.

Bug: 144537993
Test: run cts -m CtsMediaTestCases -t android.media.cts.MediaRecorderTest
Change-Id: I52345a35fb8472bd58165e9060297f8d13f3b6fc