EditTextVariations: create a IME focusable overlay for test

Bug: 228766370
Test: manual by using EditorTextVariations tool
     1) make and install EditTextVariations
     2) Enable "Settings > Display over other apps" for
        EditTextVariations
     3) Launch EditTextVariations from all apps
     4) Menu -> Show IME focuable overlay
     5) Go to home screen by gesture or pressing home key
     6) Launch any app (e.g. Chrome) and tap the editor
     7) Expect IME can show up

Change-Id: Ib4b72e0a277d8b2fb78837bed5d2e9cccd819a07