Guard mThread in ProcessRecord

ProcessRecord.mThread is guarded by mService but synchronize blocks
are missing in three instances.  This adds in the synchronization.

Only the access to mThread is guarded.  The call into the mThread is
outside the synchronization block, to avoid deadlock.

Flag: EXEMPT bug-fix
Bug: 399561948
Test: atest
 * FrameworksServicesTests:com.android.server.am
 * FrameworksMockingServicesTests:com.android.server.am
Change-Id: I4de1f60b54515434b6bb61011fa0924ccd427f5f