Remove unnecessary Binder.clearCallingIdentity()

This is a follow up CL to my previous CL [1] for Bug 26663589.

This is also a preparation CL for Bug 34838583, where the warning
shown from the method in question is to be suppressed.

The fix still works, but the Binder.{clear, restore}CallingIdentity()
block introduced by that CL is not actually necessary, because
WindowManagerInternal#clearLastInputMethodWindowForTransition() only
updates some internal variables.  No system privilege is involved.

 [1]: Ib567daa009c1139858dccadcfc6a04465ebecf36
      833bdced

Test: Manually verified that Bug 26663589 is still fixed.
       1. Flash a new OS image.
       2. Complete the setup wizard.
          Make sure a voice IME (subtype mode="voice") is installed.
          Make sure you can type with LatinIME.
       3. Connect to an external hardware keyboard.
       4. Launch an app whose window has "stateAlwaysVisible" bit.
       5. Focus in an EditText on the app.
       6. Hit the right ALT key on the hardware keyboard.
          Make sure that LatinIME shows symbol keyboard.
       7. Hit the "ABC" button at the left botton on keyboard.
       8. Tap the mic icon on the keyboard.
          Make sure that the voice IME is shown.
Bug: 26663589
Bug: 34838583
Change-Id: I88b3abb9fa88b54e4844dabd89126f5dcdda6405