Reset permission policy to default when device owner goes away

Otherwise after the Device Owner is gone, runtime
permissions might still be auto granted/denied.
I understand that there are many other policies that
we don't reset after the device/profile owner goes
away (e.g. keyguard enabled/disabled). At least now
we have a single method when we could clear the
ones that we care about.

Bug: 21889278
Change-Id: I6997655e6ef6d474bd25ae1c323eca5b17944b16