Remove @UnsupportedAppUsage from EditableInputConnection

As described in the previous CL [1], there are alternatives for app
deverlopers who somehow had to directly rely on
EditableInputConnection.

 [1]: I0c879645a84fde14dd6444bb4735f543457e43a8

Fix: 192969370
Test: atest CtsInputMethodTestCases
Change-Id: I6b60f52dfea99c262fddbc5f3d87d6f0967ae8e7