Fail on flush if no sensors are found

According to the SensorManager API, flush() is intended to return
false when no sensor is registered with the listener. Modify the
implementation to be consistent with the API contract, since no
flush command was successfully issued in this case.

Bug: 74395023
Test: Compile
Change-Id: I7e671c870aa684be54b7a5163052aac5258c0c86