Update default manifests to use master/dunfell branches

Update default manifests to use master/dunfell branches

To facilitate migrating to the upcoming Yocto Project 3.1 aka dunfell
release, update the default.xml and default-floating.xml manifest
files to use the "next" branches of the meta-agl* repositories, and
the current latest master or dunfell branches of third-party
repositories.  Third-party repositories that do not yet have dunfell
support declared in their LAYERSERIES_COMPAT variables are commented
out in the manifests.  Further iteration will be required as more
third-party repositories add dunfell branches in the run up to its
release at the end of April 2020.

Bug-AGL: SPEC-3302

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I2851cac80d61ace9bafb3062964f2d687d5d7e5c