Snap docked stack after screen rotation

- Move DividerSnapAlgorithm to com.android.internal, also move
some utility stuff into DividerUtils which is used from both
SystemUI and window manager
- When the screen rotation changes, rotate the stacks like before
but then also snap the docked stack to a valid snap position.

Change-Id: I9e1aa13f42f398a25c9016e6f20395ee212e405b