Unify hwc2_device_t and HWComposer.

HWComposer can already represent the built-in hardware composer capabilities, a
device notation is not needed. HWComposer now directly manages all displays and
owns the lifetime of all displays.  The next step is to explore whether we can
safely deprecate all hwcomposer2.h types inside SurfaceFlinger, move to HIDL
types and eventually use AIDL types.

BUG: b/77585359
Test: build, boot.
Change-Id: Id0f77b49aa441bdbf578302a04ae03b5573eb2aa