Throw for showSystemOutputSwitcher() call from privileged router

Previously, a call from a privileged router would have thrown a Security
Exception from MediaRouterService in a similar way to ag/24264289.

Bug: 192657812
Test: atest MediaRouter2Test SystemMediaRouter2Test
Change-Id: I32e27d5dfdd07f1d6dc334ab5e8fd02817006ecf
1 file changed