Fix bug with TalkBack not reading out SplitInstructionsView

This CL sets an accessibility title so that TalkBack and other tools will properly read out the instructions on SplitInstructionsView.

Fixes: 323461342
Flag: ACONFIG com.android.wm.shell.enable_split_contextual TRUNKFOOD
Test: TalkBack announces the instructions immediately when the view appears, and is not interrupted.
Change-Id: I1666f0c374d40b31d732d3cca529f03a7f2f7f28