Add setForceTranslucent API

Add this api for split screen improve enter transition.

When we start intent on side stage task, the top split root task
will go to top and current top one task will be set as invisible
then cause black screen due to split root task is full screen.
Solve this by use new wct api setForceTranslucent to make split
root task as translucent when split inactive. We need to reset
it when split active otherwise it cause flicker when back to home.

Bug: 223325631
Bug: 235029020
Test: atest WmTests
Change-Id: I28df2879345718b726af9fdb7ea54644c77ab925