Require apps to be in forground to be able to scan

After seeing the history of this change and debugging it, now OKG
doesn't initiate a scanning request and the latest documention for media
router recommends removing to remove the MediaRouter Callback in onStop
method.

- By this change we are limiting scanning to apps in the foreground
while screen is on.

- Wrap the change inside of a feature flag to help toggling it off
later if problems started to be reported.

Bug: 254634785
Test: manually && atest MediaRouter2Test MediaRouterManagerTest
Change-Id: I2d56cf3b2775f55e0e6b110c145b77d1a436a1ed