audioflinger: Update throttling logic in MixerThread

The throttling logic in MixerThread only keeps track of
time spent during sink write. However, on low tier
APs and with high sample rate tracks, it is possible
that time spent in mixing the tracks is significant.
This inturn leads to sink writes returning earlier than
expected kicking in (incorrectly) the throttling logic.

To fix this, update throttling logic to include
the time spent in mixing buffers along with the time
spent in sink write.

authored-by: Dhananjay Kumar <dhakumar@codeaurora.org>

Bug: 28672515
Change-Id: Iac4994c3084850bb93640c978b4a519db8d7951d