"Springboard" from system to new chooser.

These changes are somewhat hacky, but they allow
us to conditionally hand over to the new unbundled
chooser with minimal impact if the experiment
is disabled.

For a safer implementation, we could consider
starting with a separate springboard activity that
launches a normal (reverted) version of the
"bundled" chooser if we intend to show the UI
from the system side, but that would incur some
latency even when the flag is disabled.

Test: manual, as described in ag/16286296.
Bug: 202165481
Change-Id: I4073776b293806052e9ff66c50fea1181afd642a