System RROs can never be uninstalled. Also, enabled RRO, i.e.
RRO applied to their target packages, must not be uninstalled by
end-user because that may be dangerous to the configuration of
its target package. Disabled RROs, i.e. RRO not applied to their
target packages, are free to be uninstalled to reclaim space.
Bug: 124556507
Test: manual + `make RunSettingsRoboTests ROBOTEST_FILTER=AppButtonsPreferenceControllerTest`
Change-Id: Ib6bd2765c8cb88a5887de817a08a1541eaee0cab
c6d79cd0da