audioflinger: Clear InputSource in RecordThread::clearInput

This is for consistency with 'clearOutput' which clears the sink.

The intention is to facilitate releasing of the HAL stream which
is one of possible NBAIO_Source implementations.

Bug: 298175108
Test: atest audiosystem_tests
(cherry picked from https://android-review.googlesource.com/q/commit:49aecf0dba4b6e703d51fb2254476cad4ab60c7a)
Merged-In: I7d087836f1c57ff4872c355eac5d3ecad724b5a8
Change-Id: I7d087836f1c57ff4872c355eac5d3ecad724b5a8