Update hub background animation

Removes the linear gradient animation and replaces it with 2 radial
gradients, with their centers drawn somewhat offscreen, moving up and
down on opposite sides of the screen.

Also adds the option to remove the background entirely, and show only
the 34% scrim to darken the wallpaper/dream underneath. Found that this
is also useful to see how activities are being started underneath the
hub.

Bug: 341366334
Test: flashed device and verified animation
Flag: com.android.systemui.communal_hub
Change-Id: I26f7bae56c40668859c2b9c1f575f515ec6caab3