Fix home panel dream orientation changes

Instead of passing a fixed size to the task fragment, we now pass an
empty size. Empty sizes are special-cased to mean "fill entire parent",
and will automatically handle orientation changes.

Fixes: 323885770
Test: flashed device and verified with the flag on
Flag: ACONFIG android.service.controls.flags.home_panel_dream TRUNKFOOD

Change-Id: Ie2268cb2f65ffe2ace34a81c14918a19cce2f8d4