Synchronize access to Job.mState

Since Job.cancel() can be called at any time from another thread than
the main thread running the job, proper care needs to be taken when
checking and updating the mState member to avoid "losing" or overwriting
a STATE_CANCELED state.

Bug: 422882630
Flag: EXEMPT fixing existing synchronization issues with Job.mState
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services
Change-Id: I2e2ec38bda87892df602de61b5972948e7e7a454