Don't rewrite package restrictions unconditionally in findPreferredActivity()

We've been forcing a rewrite of package restrictions when doing preferred-
activity lookups in most cases, even when the lookup was not actually making
any changes to the bookkeeping.  Now we only do so when something has
actually changed.  Flash drives everywhere rejoice.

Bug 17334180

Change-Id: Id8f20723002b7bbcc1d82ccde55df6ef727b64ba