Window with FLAG_NOT_FOCUSABLE cant be IME target.

Any window that sets FLAG_NOT_FOCUSABLE should not be considered IME target.
IME subsystem starts input on a window when it receives focus, if window never
intended to receive focus, it should not considered an IME target either.
Also, fix the broken javadoc for ALT_FOCUSABLE_IM.

Fix: 143898978
Bug: 140641950
Test: atest WindowStateTests
      atest FocusHandlingTest
      atest WindowManager_LayoutParamsTest
      Also manually using steps:
      1. Launch gmail compose activity
      2. start typing in receipient field
      3. verify that suggestions popup window w/ FLAG_NOT_FOCUSABLE doesn't
         become IME target.

Change-Id: Ifa8e7345c2c9ad3730df86100003918b12fb533e