Verify caller before auto granting slice permission

Currently SliceManagerService#checkSlicePermission does not verify the
caller's identity. This leads to a security vulnerability because
checkSlicePermission does more than checking the permission as opposed
to simply return a boolean value -- it additionally grants slice access
under a certain condition. A malicious app can spoof the calling package
to acquire slice access.

This CL verifies the caller before granting slice access.

Bug: 208232850
Test: manual
Change-Id: I2539c9ff5ea977c91bb58185c95280b4d533a520