Update A11y strings for flex split

* Refactor to put A11y and targets we need for a given snap mode
into a new class, SplitTargetProvider
* Ideally this could be sent to DragAndDrop and other external to split
parties who need to know our split statae.
* Over time we could migrate DividerView's use of SpitLayout to another
interface.
* One issue that this does duplicate tracking of mIsLeftRightSplit since
I wanted the class to be isolated. It uses same logic as SplitLayout, but
doesn't keep it as a variable since I didn't want to add more dependencies
on config changes and what not.
* Fix logic bug in DividerSnapAlgorithm highlighted by IDE (yay!)

Test: Updated existing tests, manual
Flag: com.android.wm.shell.enable_flexible_two_app_split
Fixes: 415827083

Change-Id: I0f1a722244ae3bb8f4a2eaf5498e7a4b8e401520