Camera: Signal when all Hal requested buffers return

Depending on timing 'waitForBuffersReturned' can be
called before all Hal requested buffer are able to return.
To avoid potential timeout, check for any outstanding buffers
and signal accordingly before returning from 'returnStreamBuffers'.

Bug: 144308932
Test: adb shell /system/bin/VtsHalCameraProviderV2_4TargetTest
--hal_service_instance=android.hardware.camera.provider@2.4::ICameraProvider/internal/0

Change-Id: Idb3d8bd6fdbdfe0798c38a1b83c17f1a8379738e