Add new hub background behind a setting

Add two new alternatives for the background of the hub: a static linear
gradient or an animated gradient. The type of background is determined
by a setting, so it can be easily switched via adb without needing to
restart the device.

Command to change between settings:
adb shell settings put secure glanceable_hub_background {0|1|2}

Test: atest CommunalSettingsRepositoryTest
Flag: com.android.systemui.communal_hub
Bug: 341366334
Change-Id: I6b28b4987e45c131a6b1c5fcd32e20228e7d2ce7