Introduce an API to verify trusted caller apps by UID.

This CL introduces a new function to make sure only trusted packages
like VrCore can make use of APIs that are not intended for app use.

As a first example, this CL introduces a caller check for taking screenshots,
although any sensitive APIs should implement similar checks.

Package trust is defined by having the RESTRICTED_VR_ACCESS permission.

Bug: 34474022
Change-Id: Ib5a242d1a4e17f59b178fb1465064043613ac369