Update Flicker tests for divider dragging

Two changes to the flicker test that tests dragging the divider to a new snap point
1) The test now drags the divider further, so it can reach the 90% snap point.
2) The test now has a relaxed boundary for where app surfaces can be, since apps can go offscreen now.

Also fixes a bug where it was always referencing the inner screen bounds on foldables, instead of the screen that is currently in use.

Bug: 402767279
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: No longer fails a bunch of tests when switching to flexible split. Still passes tests with the flag off.
Change-Id: Ibc5251ed031f0416608ab5531a93d7976dfaa74c