Add depth animation to predictive back to home

Setting the depth when predictive back to home is started. This will be animated to NORMAL state by ScalingWorkspaceRevealAnim on back gesture commit.

Bug: 382453424
Flag: com.android.launcher3.predictive_back_to_home_polish
Test: Manual, i.e. testing predictive back-to-home animation on device
Change-Id: Id33f49d78aae268b7d89035cce58065f6a60a8bf