MediaMuxer:Limit scope of lock in writeSampleData

mState is the only variable that is shared in other functions. Hence
limiting the scope of autolock for checking mState only.

Bug: 157373376

Test: atest android.media.cts.MediaMuxerTest \
            android.mediav2.cts.MuxerTest \
            android.mediav2.cts.MuxerUnitTest
Change-Id: Ic5fd2205679c806a800e5e6f97579092942f204f