Add PhoneAccount check to addExistingConnection

Add a check to verify the PhoneAccountHandle of the connection being
added to Telecom via
ConnectionServiceWrapper.addExistingConnection. This allows us to be
sure that the existing connections being added are from a registered
PhoneAccount and that the Package and User match what is expected in the
PhoneAccount.

Change-Id: I12c5f1b046a4ca919303ace205a9ddd6c7092650
Fix: 28800963