AudioFlinger: use std::map for mHwDevice

Also AudioHwDevice::hwDevice() now returns a const ref
for improved efficiency.

Flag: EXEMPT bugfix
Test: atest CtsMediaAudioTestCases
Test: atest CtsNativeMediaAAudioTestCases
Bug: 422168293
Change-Id: I568ca4c29dda3185634aa4019b1c51db825fee3a