Reenable optional permissions parsing and expose APIs

Modify the package parsing code to understand optional permissions
(android:required="false"). This is essentially a rollback of
e8241200 with some minor changes.

Expose the requestPermission API to third party apps. This allows
an app to request an Intent which, when passed to
startActivityForResult, will prompt the user to approve permissions
for an app.

In the event we decide to not launch with this feature, this change
can be rolled back.

Change-Id: Ie3626deae9b16e510323f94a2c80377f7c84b26f