Simplify/correct Taskbar insetsRoundedCornerFrame calculation

Now we base it on the current tappableElement insets, which will only be
> 0 for the unstashed, persistent taskbar which draws rounded corners.
This inset also stays stable during temporary stash transitions such as
when the IME is showing.

Flag: ENABLE_TRANSIENT_TASKBAR=true, ENABLE_TASKBAR_PINNING=true
Test: in split screen for both transient and pinned (persistent)
taskbar, ensure split screen divider bottom is drawn appropriately
Fixes: 278028503

Change-Id: Iab56c5dc8f9fc81c34aa01f41b5212dd5c6fd7f0