[Dual Shade] Remove the DualShade flag and remaining references to it.

The new way to enable Dual Shade is via the following ADB command:
`adb shell settings put secure dual_shade 1`

(pass 0 to disable.)

To check whether dual shade is enabled via ADB:
`adb shell settings get secure dual_shade`

Fix: 388793191
Test: Unit tests still pass.
Flag: EXEMPT removing com.android.systemui.dual_shade
Change-Id: Iba4ea07f7ce397f21e6b13a5c61f95fdaeb33431