Revert new logic around FLAG_ALT_FOCUSABLE_IM

This broke several apps because it means certain NON_FOCUSABLE windows
are now layered wrong w.r.t the IME, and also no longer get IME insets.

Originally, this was introduced because the IME target was used to
compute which window gets control, but this is now computed based on
the window actually connecting to the IME as reported per IMMS.

Reverts I941571c97145d77b0a59d030cf2a8c8318f3b59f.

Bug: 143898978
Bug: 140641950
Bug: 145812508
Bug: 141738570
Bug: 144619551
Fixes: 159438771
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 even thoug the suggestions popup
         window w/ FLAG_NOT_FOCUSABLE becomes the
         IME target, control and insets still work.
Change-Id: If451276b1a8c485ec88965d8d30ec8fd3836620f