audio: Clarify comments on AudioPortMixExt fields

Specify that '0' means 'unlimited' for
AudioPortMixExt.maxOpenStreamCount, and 'all opened streams'
for maxActiveStreamCount.

Bug: 205884982
Test: N/A
Merged-In: Ia212fef049778a85046e3333faa0ddf8fe167c1f
Change-Id: Ia212fef049778a85046e3333faa0ddf8fe167c1f
(cherry picked from commit 52ca76ed)