homescreen_git: Bump SRCREV to support 0.0.21 AGL compositor version

This SRCREV bump update is required to add support for the 0.0.21 AGL
compositor version. With it, we change the default homescreen with
multiple surfaces to just using a single background surface and
explicitly set an activation area.

In more detail the following changes have been added:

- 444b2a68a homescreen: Only keep the multiple surfaces approach in CI
- 4c7852e6f homescreenhandler: Avoid using a stale wl_output
- 7ff53ae1b meson.build: Bump to the latest agl-compositor version
- bb4c83e29 homescreen: Add support for defining an activation area

Change-Id: I23dfa8c8304b0493f29f38c2afb4a4837980a357
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>