Add push to talk support to homescreen

Add push to talk support to homescreen

Reworked version of Alexa specific changes from ICS to add push to
talk button for voice services to homescreen media area.

v2: change config.xml to audiomixer

v3: reworked to not be Alexa specific:
- Now use the default voiceagent if available, instead of hard-coding
  Alexa usage
- The Alexa logo for the button has been replaced with a generic
  microphone icon derived from the radio application's launcher icon.
  This is a placeholder until a new icon is provided by LF graphics
  team. Meeting any Amazon requirements around Alexa chrome is now
  envisioned as being provided for with a TBD voiceagent API
  enhancement.
- The QML for the PTT button has been moved to MediaAreaBlank.qml,
  which seems a more logical location for it ATM.  It is likely that
  the MediaArea QML should be simplified in a future change, as it
  currently contains a signficant amount of unused code.
- The PTT button has been moved to the left hand side of the media
  area, as this seems more sensible if demonstrating driver usage.
- The delay on fade-out of the master volume slider has been lowered
  to 3 seconds from 5, with the PTT button present it started seeming
  excessive during testing.
- Some extra debug messages have been added to make tracking the
  voiceagent state more straightforward.

Bug-AGL: SPEC-2764,

Change-Id: I2052e345baaf4306e8e3f27a01bc6940f4d27d88
Signed-off-by: Naveen Bobbili <nbobbili@amazon.com>
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Signed-off-by: Scott Murray <scott.murray@konsulko.com>