SF: use shared_ptr to track hwcLayer

Add a wrapper class and use std::shared_ptr to track instances
of hwcLayer.  This is necessary because the calculation of
data for HWComposer is done in Layer and the processing of that
data is done in SurfaceFlinger, and we need to make sure that
the hwcLayer is not destroyed by Layer while or before being
used by SurfaceFlinger.

Merged-in: Icc7c002b84aaded1966d24aa8f849b47f51d2efb

Test: Compile/Boot
Change-Id: Icc7c002b84aaded1966d24aa8f849b47f51d2efb