drm_hwcomposer: implement the safe handling of layers

drm_hwcomposer: implement the safe handling of layers

This is a sweeping change to discard our usage of struct hwc_layer_t outside
hwcomposer.cpp. That was a dangerous struct that was a source of many of our
errors. Replacing it with safer RAII-style classes reduces the amount and
complexity of our code.

Change-Id: I580cafdf89bd1e7e6583f3073858b8e78e6018ba