Move EditableInputConnection into com.android.internal.inputmethod

EditableInputConnection has been one of the most important and widely
used InputConnection implementations.  While having it under

  com.android.internal.widget

would still make some sense, it'd become more easier for the IMF team
to keep improveing it if EditableInputConnection is moved into

  com.android.internal.inputmethod

Anyway, EditableInputConnection has never been a public API hence just
moving its package should have no impact on app compatibility.

Bug: 192412909
Test: atest CtsInputMethodTestCases CtsWidgetTestCases:TextViewTest
Change-Id: I87974f779dbda60dfb79331cbe1ec975c475c695