hwc: Refactor windowboxing feature

1. Remove AIV video mode composition stratergy and reuse video only
   composition stratergy to achieve the same functionality.
2. Drop all non AIV layers from the external list at the start of MDP
   composition prepare and fall back to video only composition.

Change-Id: I34760e2df57cfbb923a6be0182e632c9ddd6aa07
4 files changed