Make ImageWriter thread safe

The various functions in ImageWriter were not protected by a lock, which
led to segfaults under certain race conditions.

This CL adds a mutex to prevent ImageWriter methods from executing while
ImageWriter is closing. This also prevent multiple calls to close from
executing simultaneously.

It also adds a simialar mutex to WriterImageSurface to prevent
simultaneous calls to close().

Bug: 244262872
Test: mediaframeworktest:com.android.mediaframeworktest.stress
Change-Id: I44976e5a36154ce98f64e2982865b2defcec7f5e