[Dual Shade] Add new setting for enabling Dual Shade.

This setting will soon replace the feature flag:
`com.android.systemui.dual_shade`.

It is a secure setting (tied to the user) and will be backed up.

To enable the setting via ADB:
`adb shell settings put secure dual_shade 1`

To check the setting via ADB:
`adb shell settings get secure dual_shade`

Test: N/A - adding the constant
Flag: EXEMPT constant is not yet used
Bug: 388793191
Change-Id: Ica8002cb61c76c8ece812515e288f14e4b3e1537