Ensure scan timeout is reset if new scan started

Add the logic that aligns with the existing comment, by removing pending
MSG_SCAN_TIMEOUT events for the given client when a new one is
scheduled.

Test: atest BluetoothInstrumentationTests
Bug: 300916771
Bug: 316266556
Change-Id: I0bcdca776ba1869038e01d773282b86da1825190