Fix LaunchAppShowImeAndDialogThemeAppTest flakiness

Using tapping DialogThemedActivity may possible to get unexpected focus
change to hide the keyboard, also DialogThemedActivity self doesn't set
android:windowSoftInputState=stateUnchanged, that affects the test
verfication that we expeting IME layer should persist visible.

Fix it by adding stateUnchanged softInputState flag, and simply using
UiDevice#pressBack to dismiss dialog without touching unrelated focus
change.

Fix: 215884488
Test: atest LaunchAppShowImeAndDialogThemeAppTest \
        --rerun-until-failure 20

Change-Id: I4704dfa531836423dafc3f3ece1d3b80584ae7fa