Prevent doubling up on split placeholder insets

This patch makes it so that the split placeholder uses only one inset for buffering the top of the icon: either the default split placeholder inset, or the device's built-in screen inset.

Currently, when we determine the size of the split placeholder, we add the placeholder's default inset to the device's built-in screen inset (camera cutouts, etc.). On some devices, this causes a very large combined inset, and it doesn't leave enough room for all the other UI elements, especially when 3-button mode is active. This patch cuts out one of the insets by selecting only one of the insets to respect (the larger one).

Fixes: 241164191
Test: Manual, checked all emulated inset types on a variety of screen sizes
Change-Id: Ica0e791bf4b8b757ad3fb87ab1cf3105b97a61dc