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.
Certain apps try to manually synchronize app window to IME insets by
using ADJUST_NOTHING in main window and introducing a transparent window
on top with FLAG_NOT_FOCUSABLE & ALT_FOCUS_IM to listen for IME insets.
We will support this use case until new focus management takes care of
it.

Fix: 143898978
Bug: 140641950
Bug: 145812508
Bug: 141738570
Bug: 144619551
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: I941571c97145d77b0a59d030cf2a8c8318f3b59f