Fix the pointer position when displayed in RTL

When displaying in RTL, the pointer translates from the opposite
side of the screen than the code assumes. This CL changes the math
for the pointer position when displaying in RTL.

Test: manual - test landscape / portrait on tablet / phone in LTR
               and RTL: have a couple of bubbles, expand the stack
               and select each bubble ensuring that the pointer is
               centered on the selection.
Bug: 230579507
Bug: 233950592
Change-Id: I71a6f64a43c1b7bb5e0a30773d4cc5ae55c63b6a
Merged-In: I71a6f64a43c1b7bb5e0a30773d4cc5ae55c63b6a