Ensure previous batched input is consumed when enabling batching

Handles a race where disabling batching with pending batched input
events may leave pending batched input unconsumed indefinitely if
batching is enabled again soon after - before the task to consume
pending batched events is run. In that case input consumer would be left
in state waiting for the input receiver to consume pending batched
events, and all incoming batched evets would be left unprocessed (until
a timeout).

Bug: 430596235
Test: None
Flag: EXEMPT bugfix
Change-Id: I10ff115a9d42019c61873bb76072df6bd1b4b301