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
Change-Id: I7d087836f1c57ff4872c355eac5d3ecad724b5a8