OMS: harden permission checks

The IOverlayManager.aidl interface is protected by certain permissions.
The overlay manager implements the permission checks by calling
enforceCallingOrSelfPermission, but this method needlessly includes the
permissions of system_server in the check.

Harden the permission checks by switching from
enforceCallingOrSelfPermission to enforceCallingPermission.

Bug: 78809702
Test: atest OverlayHostTests OverlayDeviceTests
Change-Id: I5851dd1683adf644ea8e5a58dce4d7377664342e