audio: Fix default remote submix HAL implementation and VTS

The implementation had duplicated code in 'transfer', which already
present in 'outWrite'.

Cleaned up delay calculations and logging.

Fixed the VTS to send 'prepareToClose' before attempting to join
the worker. Otherwise, the worker could be stuck on a blocking
operation due to inactivity of the other party and join would never
happen.

Bug: 302132812
Test: atest VtsHalAudioCoreTargetTest --test-filter="*AudioModuleRemoteSubmix*"
Change-Id: Id8455eb12d1d2999dc0bc7b64f0d70a61a177598