InputDeviceSensorManager: Fix broken input device sensor flushing logic

Sensor flushing logic was broken, because the
InputSensorManager#flushImpl() called InputSensorManager#flush()
instead of InputDeviceSensorManager#flush(), which in turn called
InputSensorManager#flushImpl(). This was an infinitely deep mutually
recursive pit.

Fix this by renaming the local implementation to flushInternal.

Bug: 290254916
Test: atest InputDeviceSensorManagerTest
Change-Id: Ica72ec25956a1b27387d791957474b8e98d423ca