capengine: share preferred input logic with default engine

Move getPreferredAvailableDevicesForInputSource to EngineBase so it
can be shared by both engines(default and cap).

This is for when settings app calls setPreferredDeviceForCapturePreset()
the engine should give the preferred input higher priority to use.

Bug: 436398891
Test: Join Meet call via Chrome.  Visit sound settings to select audio
input and verify the real capture mic is changed accordingly.
Flag: com.android.media.audioserver.cap_engine_preferred_device_improvement

Change-Id: I70b0c55c9e7287b30fa7edb20bb961314b9e45f0