Fix cancelScanRequest() scanning state update logic

Before this change, the method would not turn off scanning in
system_server when a screen-on scan request was released. As a result,
proxy routers would unknowingly keep scanning forever even after having cancelled
their scan requests.

Bug: 331202666
Test: atest MediaRouter2HostSideTest ProxyMediaRouter2HostSideTest
Change-Id: I63563f38b7d53e608782e651150242519a55f5ce
Flag: com.android.media.flags.enable_screen_off_scanning