Fix race condition in ActivityThread.completeRemoveProvider()

This should solve the problem when exceptions like below are thrown:
java.lang.IllegalStateException: unstableCount < 0: -1
android.os.Parcel.readException()
android.os.Parcel.readException()
android.app.ActivityManagerProxy.refContentProvider()
android.app.ActivityThread.releaseProvider()

Change-Id: I97ca9f7f0b57221bc3aa41727936292cc79b92c9