policy_hal: remove unused function isVirtualInputDevice

- isVirtualInputDevice is a static member function and
  its implementation should be modified in base class only.
- Remove isVirtualInputDevice in policy_hal and modify
  APM_AUDIO_IN_DEVICE_VIRTUAL_ALL in base class to support
  FM recording over A2DP headset.

Change-Id: Ib0527cfdff170367491feb62b117815558e139e1
CRs-Fixed: 581377