Do not clear a shared user's permissions on an app install.

When regranting permissions for an app during an install if
that app is in a shared user we should not clear the permissions
as the permissions for the shared user are additive and go away
when apps requesting them are uninstalled.

bug:20050689

Change-Id: I82aa70669fc25a45e7020a1545b093db5525f5cf
2 files changed