Fix placement bug with split staging icon in RtL

Fixes a bug where switching the phone to RtL caused Overview's split staging icon to be placed incorrectly.

The bug was caused by the fact that FrameLayout margins were not working consistently in RtL. This may be another bug; more investigation required (b/201920967).

This fix makes it so that the icon is positioned using setX and setY rather than margins.

Fixes: 227372971
Test: Manual
Change-Id: I6e5371d20e8e2e4e6f41ba93d0dfa36ed085c103