Manage scan requests in MediaRouter2Manager

Before this change, MediaRouter2Manager (singleton) does not
protect clients against independent changes to the scanning
state, which means one client can stop the scan while another
needs scanning to go on. After this change, MediaRouter2Manager
keeps count of the calls to request scanning, and will only
request scanning from the system server while there is at least
one active scan request (via registerScanRequest).

This change does not affect MediaRouter2.start/stopScan.

Bug: 232812007
Test: atest CtsMediaBetterTogetherTestCases
Change-Id: I8d80872d7c55791b9a429180da1c68af50ae13b7