Runtime permissions: per user permission tracking.

Before all permissions were granted at install time at once, so the user
was persented with an all or nothing choice. In the new runtime permissions
model all dangarous permissions (nomal are always granted and signature
one are granted if signatures match) are not granted at install time and
the app can request them as necessary at runtime.

Before, all granted permission to an app were identical for all users as
granting is performed at install time. However, the new runtime model
allows the same app running under two different users to have different
runtime permission grants. This change refactors the permissions book
keeping in the package manager to enable per user permission tracking.

The change also adds the app facing APIs for requesting runtime permissions.

Change-Id: Icbf2fc2ced15c42ca206c335996206bd1a4a4be5