Use flutter-auto activation area support

With the geometry fix in place for flutter-auto's activation
area support, update the Flutter homescreen JSON configuration
files to use it, and drop the custom Flutter weston.ini that
was being used.

Bug-AGL: SPEC-5038

Change-Id: I2e5fe3b8e54f49a36c2286e1a78ffc081b90f6fa
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29582
ci-image-build: Jenkins Job builder account
Reviewed-by: Marius Vlad <marius.vlad@collabora.com>
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account