Add config variable for targeting glanceable hub

Glanceable hub will now be targeted with an Android config boolean that
is overridden in Pixel-specific configs. This supersedes the previous
SysUI config boolean.

The rollout of hub onto mobile will be controlled by the
com.android.systemui.glanceable_hub_v2 flag.

Bug: 378113263
Fixed: 378113263
Test: atest CommunalSettingsRepositoryImplTest
      manually logged on device to verify config flag is enabled
Flag: com.android.systemui.glanceable_hub_v2
Change-Id: I68b4c35904dac9040ccff8ca5fce7da0c187ce84