Allow for late audio module discovery in APM

Extract code that loads hardware modules and fills out
mAvailable{Input|Output}Devices into a reusable function.
It is used both during APM initialization and on
receiving a notification that a new HAL service has
been discovered.

Bug: 149854039
Test: atest audiopolicy_tests
Change-Id: Ifb7f0b61b06a0609802e63dccb26f3472328ef48
Merged-In: Ifb7f0b61b06a0609802e63dccb26f3472328ef48