Add getter for system session overrides

This is necessary in case SysUI crashes and needs to restart while a
mirroring session is already ongoing.

Bug: b/396394220
Test: atest CtsMediaRouterTestCases
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: Ic3866066124f5b4c63dd4569e419a0c1415c702b