Update policy to not silence ongoing phone call when assistance was triggered

This behavior (assitance unavailable but phone call input not muted) aligns
with cellular phone call + assitant concurrent use case.

Also make a few local variables const.

Bug: 337285234
Test: atest CtsMediaAudioTestCases
Test: VOIP call with pixel
Change-Id: I06310126ee875e14bda057b0b688aa0b5335626d