audio: Fix remote submix behavior with multiple inputs

Align the behavior of the new implementation with the legacy
implementation in the case when the client opens multiple
input streams on the same remote submix device.

Reduce the HAL buffer size to match legacy behavior.

Bug: 264712385
Test: atest CtsVirtualDevicesTestCases --test-filter=".*VirtualAudioTest.*"
(cherry picked from commit a83c8a55)
Change-Id: I6f5e6a2c133059c66269b1bdd8a30392db282c4d