DeviceConfig flag to guard delegate chooser flow.

This replaces the ad-hoc hard-coded flag that previously
guarded the delegation flow, and updates the config to
point to the new delegate component. Note this is still
the Phase 2 ("Delegation Trials") architecture from
go/sharesheet-unbundling-phases which delegates to the
new component only for the final step of dispatching the
user's selected target. When the new component takes
responsibility for displaying the Sharesheet UI (Phase 3)
we'll use the same DeviceConfig flag to guard the new
logic to delegate out from earlier on in the system
activity lifecycle.

I was able to test this manually (once I eventually
figured out how to get the new unbundled APK installed),
by going through Sharesheet and confirming that the
delegate activity (at the new path) is only invoked after
overriding the flag by:

adb shell device_config put systemui use_delegate_chooser true

An upcoming CL will add the server-side config that we
can use to control this experiment.

Bug: 202166034
Test: manual (as described above)
Change-Id: Ie3eb5bca6b768383156293cddbf6932df2cd3a64