audio :Make isVirtualInputDevice as protected method

- startInput is overridden in AudioPolicyManagerALSA and is
  making a call to isVirtualInputDevice which is private in
  AudioPolicyManagerBase.
- Make isVirtualInputDevice as protected in base class so
  that it can be called from the derived class

Change-Id: I7e499b0c06f9412bc191179d497298cd57af2c3a
CRs-Fixed: 591788