Ensure toolbar popup respects left system bars

The current x position calculation only respects the right side of the
display frame, not the left side. This can cause the popup to sometimes
render under a system bar attached to the left side of the screen.

Bug: 328290309
Test: manual
Flag: NONE bugfix
Change-Id: I81be5f6d32fb0ac318f1efc81b776824e0951427