VolumeControlService: Add AICS support

Patch adds internal VolumeControlInputDescriptor which keeps
informations about external inputs on the remote side.

Bug: 291285174
Bug: 361263965
Test: atest VolumeControlInputDescriptorTest
Flag: com.android.bluetooth.flags.leaudio_add_aics_support
Change-Id: I28f9c9b8ec471d3b29975e1b343517b99e40c6dc