fix(magnification): ensure the old path is cleared when hiding the fullscreen border

In AccessibilityController drawOrRemoveIfNeeded, if new alpha == 0 then the border surface will hide without clearing the current border path. So if next time show the border surface with different bounds, the old border path would also be showing. Therefore, we cache the last usedalpla for drawing, then if new alpha == 0 and cached alpha != 0, which means currently there exists a border path, we would need to clear the path first then hide the surface.

Bug: 325863281
Flag: NA
Test: manually
Change-Id: I81a16edcdd549dd42cf740ccce336b52060c4b5b