Homescreen push to talk button addition

To allow demonstrating speech recognition, the Homescreen application needs a reworked version of this change applied:

https://gerrit.automotivelinux.org/gerrit/c/apps/homescreen/+/21182

Some discussion on this at the recent integration session and afterwords has resulted in the following desired features:

• Homescreen should always pull in vshl-core and not use a build time flag to optionally do so.
• Homescreen should not display the PTT button if no voiceagent is available.

To accomplish this, some accompanying modification of agl-service-voice-high is required to make voiceagents optional in addition to reworking the original Homescreen change.