Resizing to invalid bounds causes task to jump to fullscreen

When resizing the task to a negative width/height, the task becomes
fullscreen. This change stops shell from sending invalid bound changes
(changes where width/height is less than minimum task width/minimum task
height) to core.

Bug: 269660668
Test: Flashing device and manual testing
Change-Id: Ie0f133be03d3201cc4f6b266bd7d27af624571e4