Fix bugs when system apps reclaim permissions.

Recently we started letting system apps always take precedence over
third-party apps when defining permissions.  This change fixes that
logic to claim the permission immediately, instead of delaying until
after the next reboot.  (Permissions are always reevaluated after
each install.)

We also tighten the constraints slightly to prevent two system
apps from fighting over a permission definition; the first system
app to claim the permission wins.

Bug: 17526617
Change-Id: I49686407f5e99322bc511795c653c5d702becd9d