Maintain the position of accessibility floating menu

* Use percentage of X-aixs and Y-axis as last position, so it could be
restored at approximately position when device screen size changed.
* Fine-tune AccessibilityFloatingMenuViewTest

Bug: 183342667
Test: atest AccessibilityFloatingMenuViewTest PositionTest
Change-Id: I58ed608a03bc20f15a9d0852c95907107516c9ba