Make DialogThemedActivity main window is not focusable

In case the window may become to IME input target when dismissing the
dialog window to mistakly hide the IME when getting the IME insets
control.

Also, add the SystemBars' insets visibility string on the dialog for the
test to easier track and verify the insets visiblity before thetest.

Bug: 227142436
Test: atest LaunchAppShowImeAndDialogThemeAppTest
      --rerun-until-failure 10
Change-Id: I96b17c78d4bd211a930bc0e4a083168e83f8aae9