Fix scan request leak in RouterInfoMediaManager

If a client of the class calls requestScanOnRouter twice in a row, the
first second scan request will never be released because the scan token
is leaked.

Bug: b/352271568
Test: atest SettingsLibTests
Test: Manually by adding logging and chasing scan requests.
Flag: com.android.media.flags.enable_screen_off_scanning
Change-Id: I7ad57076fff6c62f33898b06da6b5f094c451643