Add application launcher support

Add application launcher support

Changes:
- Add required agl-shell and applauncher gRPC API source and
  generated files.
- Remove unused deprecated databroker gRPC API files as cleanup.
- Add app launcher helper object and associated RiverPod provider.
  The implementation is based on code from the old Flutter
  homescreen.  There will likely be follow up work to use the
  recent changes to the agl-shell gRPC API to handle display
  setting for applications needing remote displays.
- Wire up application enumeration and starting in the app page.
  A placeholder generic application icon has been added that will
  be used for the external applications for now, as the SVG icons
  being used with the old homescreens are not really suitable.
- Wire up activating the homescreen again if the bottom panel is
  touched after an external application has been started.

Bug-AGL: SPEC-5026

Change-Id: I01de4760cfd098d3b5f2e6843ad9103a8ea87935
Signed-off-by: Scott Murray <scott.murray@konsulko.com>