Fix issue with camera overlapping split staging UI

Fixes an issue on some devices where initiating a split in Overview would cause an icon to be placed partly under the camera cutout.

Fixed by adding some padding to FloatingTaskView#mSplitPlaceHolderView, so that it always respects the device's insets.

Fixes: 220997064
Test: Manual
Change-Id: I7270bde3320f562519ea8e36a38e49609e0d6cd7