audio: Fix AudioRecordTest#testTimestamp CTS on CF

After experimenting, it seems that the input poll
is more stable after the PCM input on CF has just
been opened. Since calling 'flush' on an input
stream puts it into the 'STANDBY' state, explicitly
call 'StreamAlsa::standby' after 'flush' in the
primary input stream implementation.

Bug: 340899868
Bug: 362852052
Bug: 372951987
Test: atest CtsMediaAudioTestCases
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ic1df6835ce00323ca3f0905ea46d3bc151e05fc4