Dump WindowHierarcy on getUiObject failure

This adds an additional dump of the window hierarcy when getUiObject
fails, to complement the dump of the screenshot on failure. This should
point why the screenshot shows the button being visible, but UiDevice
doesn't find it.

Flag: EXEMPT test debug
Bug: 395783236
Test: atest InputMethodServiceTest#testBackButtonClick
  InputMethodServiceTest#testBackButtonLongClick
  InputMethodServiceTest#testImeSwitchButtonClick
  InputMethodServiceTest#testImeSwitchButtonLongClick
Change-Id: I210249f43c6c53f3f185f17645e563732e13e072