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,

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>
Change-Id: I398bf7aebc5c9b459b1fce94511eee3698c08347