A2DP: HALs switching between software and offloading sessions

This change would open both audio providers of A2DP software and
offloading after initialized, and is based on the codec to select which
provider should be used.

* When A2DP source is starting up, open both software and offloading
  audio providers.
* When setup the codec, choose the active provider by its codec
  capabilities and offloading preferences.
* API for others to query whether the provider is running under
  offloading or software data path.

Bug: 143922003
Test: manually
Change-Id: I322554f6549e5fdb9e0914d0972dd5cd64d568a0