Feature: showAtCenter

[skydoves]

Guidelines

• Shows the balloon over the anchor view (overlap) as the center aligns.
• Even if you use with the [ArrowOrientationRules.ALIGN_ANCHOR], the alignment will not be guaranteed.
• So if you use the function, use with [ArrowOrientationRules.ALIGN_FIXED] and fixed [ArrowOrientation].
• @param anchor A target view which popup will be shown with overlap.
• @param xOff A horizontal offset from the anchor in pixels.
• @param yOff A vertical offset from the anchor in pixels.
• @param centerAlign A rule for deciding the alignment of the balloon.