a11y: panel - Allow drag and snap to nearest side

 * Add drag feature to allow users move the panel freely on the
   screen.
 * Panel snaps to left/right side when user lets go.
 * Maintains vertical position during snapping to the sides.

video: http://shortn/_1pQ3yEE7Ng

Bug: 395884085
Test: AutoclickControllerTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: Idd73de95b729ebc4374d0aee1437966d89637dff