Wait until DragDropActivity is ready for input

Bug: 294251373
Test: atest android.server.wm.DragDropTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:62ebaca14b17c26b6cbac9ec9cd208b194ef808d)
Change-Id: If2cc0f143286eb415a14abcb68a56c8e49eed6cd
Merged-In: I2e1074e0cae30817cfff2fd2cfcf9971d356c00a