ContentProviderRecord: Remove external process reference when finalized

ContentProviderRecords is removed from provider map by activity manager service
when the process hosting the content providers terminates and this causes GC
to free the content provider records but the reference to external process
using the content provider isn't cleared. Fix is to clear the reference and
unlink for death notification.

Change-Id: I67fd10cee57b02257792d0c9b4569fe11dea7648