EGL BlobCache: Synchronize access to deferred write status

Protect access to mDeferredWrites with a mutex. It is updated
by the worker thread upon completion.

Test: pubg_mobile_launch ANGLE trace
Test: /data/nativetest64/EGL_test/EGL_test
Test: /data/nativetest64/libEGL_test/libEGL_test
Bug: b/271975248
Change-Id: Id278f4677a72f2d981a9400ee871bdcc1b0168ef