blast: protect destruction of callback thread

Wrap the creation and destruction of TransactionCompletedThread's
mThread in a mutex. This will prevent the race condition between
the creation of mThread and mThread.join().

Test: libsurfaceflinger_unittest
      Transaction_test

Change-Id: Ic448872cd5418ce7885b8c2e52a7f1bf02afd772
2 files changed