Use InputInjection util for InputMethodServiceTest

In testShowHideKeyboard_byUserAction we were using UiDevice to search
for the EditText, and to click on it, despite having a reference to the
View readily available. This makes the test more robust by using the
InputInjection utils to directly click on the existing reference.

Flag: EXEMPT testfix
Bug: 394328311
Test: atest InputMethodServiceTest#testShowHideKeyboard_byUserAction
Change-Id: Ib2f51b16126467a4f1a857657c5e6086fb63159c