Sanity-check erroneous backup agent instantiations

Two distinct changes:

Fix a bug seen in the wild where a newly-launched application will be
spuriously asked to instantiate a backup agent.  What was happening
there is that some Activity Manager state was being left stale in certain
circumstances, and then in combination with app uninstall / install, there
could be a case where uid reuse wound up looking like an app identity
match.

We now positively verify before instantiating the agent that the intended
backup target package is uid-compatible with the app process that the
instantiation was requested of.  The incomplete bookkeeping in the
Activity Manager has also been tightened up, and the Backup Manager is
more aggressive about cleaning up pending operations pertaining to
apps being uninstalled.

Bug 5874010

Change-Id: Ic389f4a96c9dcd0ba6b3962b579084033d8ae9f8