Split callback registration and route scanning in InfoMediaManager

This is prework for allowing clients to receive routing updates from the
platform without having to scan for routes. For example,
MediaDeviceManager populates the device chip information in the UMO,
which needs to know the current connected device, but has no use for
scanning.

The root limitation we are trying to address is that
InfoMediaManager couples callback registration to route scanning, so it
forces all clients interested in receiving routing state updates to also
scan needlessly.

This is a non-functional change.

Test: atest com.android.settingslib.media
Bug: 332515672
Change-Id: I2d238cc8761c5429458ae33dc0fd034c6cf11e9e