Protect against crash surfaced by monkey, where it seems that the window
of the dialog is no longer present. If not present, don't need to post
dismiss runnable.
Bug: 124327419
Test: adb shell monkey -p com.google.android.GoogleCamera -s 999 -v 20000
Change-Id: Ic03b8501bb73725e04544fcb2397532be531988e
d0de1cd170