Add MR2 listener for the list of apps with system session overrides

A system session override is an alteration on how system media is routed
for the app with the override. Per-app dynamic audio routing policies
are an example of overrides. The new listener enables components (like
SysUI) to know which applications have a system session override. And
this, in turn, allows said components to create a Proxy MediaRouter to
control said apps system routing session.

Bug: b/396394220
Test: atest CtsMediaRouterTestCases
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: Ia1fda27b0b6402621e37e669283dbbb3763403b1