AudioPolicyManagerBase : MUTE_TIME_MS reduced to 500ms

 - Some streams are muted while the audio path
   is switched for different device switch like
   BT, Headset, AOS and HDMI etc. Audio drop is
   observed for more than 3-4 secs. So tuning
   this macro for better results.

Change-Id: I8bb0b6f5eb5e526b755fce97f31a3f8f6dd22064
CRs-fixed: 572654
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