Fix LayoutTransition All Apps divider logic for RTL.

We were checking the wrong index (off by one), leading to a crash if the divider is already in the Taskbar at the correct index. However, we do need to add it after the expected index if the divider isn't there, otherwise it will push the icon that is at that position to the right of the divider (it should be to the left).

Flag: com.android.launcher3.taskbar_recents_layout_transition
Bug: 343521765
Bug: 368522274
Test: go/testedequals
Change-Id: I0263f704728286b53da5b45828784b20aaae31ce