Optimize Permissions Hub by not loading non-platform permission groups.

Added a flag when loading permission groups that controls whether or
not to load non-platform permission groups.  Some uses, including
Permissions Hub, only use the platform permissions, so we can save
time by ignoring the others.

On my dev phone this reduces the Permissions Hub loading time by about
a quarter.

Bug: 124052170
Test: Load Permissions Hub, see it is faster.
Test: Ensure App Permissions and search indexing still work.
Change-Id: I1489d227342e115464363cb78e914ce78ad88d5e