[Dual Shade] Allow the new Dual Shade setting to enable Dual Shade.

Temporarily, we allow the dual_shade aconfig flag to enable Dual Shade
in addition to the setting (i.e. enabling either the setting or flag
will enable the feature). This helps avoid breaking the many unit tests
that still rely on the flag. Once all code references to the flag have
been removed, we'll remove the flag as well.

BONUS: Adds convenience methods in Kosmos for enabling each of the shade
modes.

Test: Unit tests still pass.
Flag: com.android.systemui.scene_container
Bug: 388793191
Change-Id: I6928f2625b175ba25a3be2e1a31a4f8e8ef32ee1