Remove @UnsupportedAppUsage from InputMethodManager#getInputContext()

We are mostly confident that this @UnsupportedAppUsage was a false
positive.

Fix: 194323038
Test: presubmit
Change-Id: Id83ce2bd5896e953c824f7eaf28b235859123ab0

core/java/android/view/inputmethod/InputMethodManager.java

@@ -1208,8 +1208,15 @@ public final class InputMethodManager {
         return mClient;
     }
 
-    /** @hide */
-    @UnsupportedAppUsage
+    /**
+     * No longer used.  Do not use.
+     *
+     * TODO(b/192412909) Remove this method.
+     *
+     * @deprecated Was kept due to {@link UnsupportedAppUsage}.  Will be removed soon.
+     * @hide
+     */
+    @Deprecated
     public IInputContext getInputContext() {
         return mIInputContext;
     }