Fix heads up/ambient content inflation w/ groups.

This CL fixes lazy content inflation for alerting notifications in
groups by inflating them on the fly if we would transfer to a
notification that does not have its content inflated. We introduce a
helper class here to explicitly deal with the transfer logic,
refactoring a lot of the GroupManager code out.

This removes the previous workaround to always inflate heads up +
ambient views.

Bug: 111809944
Fixes: 111809944
Fixes: 117933032
Fixes: 117894786
Test: runtest systemui
Test: manual, posted groups that trigger a transfer and a transfer back.
Group alerts behaved as expected.
Change-Id: I9b5ec4c8bdeea20707874d90213dcd1d22d8b503