Remove IME surface when hidden

When IME is no longer shown, notify IMS to hide the window surface.
This reclaims the memory allocated to IME window surface.

Fix: 150163310
Test: atest CtsInputMethodTests
      Manually:
      1. Open any app that has editor  and run
         adb shell  dumpsys SurfaceFlinger | grep Total
      2. Note down total memory
      3. Tap on editor and IME shows. Note down memory
         using command in #1. It should go up by ~27MB
      4. Hide IME and note down memory again.
         It should revert to original value in #1.

Change-Id: I6c2a13dd40b22d3fd62b4763ba369992e5ff1138