AudioPolicyManager: keep track of activity for HW Audio Patches

In order to allow volume management for all volume sources, this
CL adds Source Clients even in case of HW AudioPatches.
It uses the mixPort declared to reach the sink involved in the
HW Audio Patch to add this client.

Test: Manual: build & call & plays with HW / SW AudioBridges.
Auto: (Non-regression on emulator)
adb shell ./data/AudioPolicyEmulatorTests --gtest_filter=*AudioPatchTest*
adb shell ./data/AudioPolicyEmulatorTests --gtest_filter=*AudioSourceTest*

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: Iaf44db2fd9a90dde9ef8160869af73983a5feae4