SF: Separate Hwc2::Composer into interface and impl

This makes the android::Hwc2::Composer substitutable. In this case the
intent is to allow a mock::Composer GMock class to be defined.

Hwc2::Composer now is a pure interface class. Hwc2::impl::Composer is
the normal implementation.

Also included is another minor change to allow HWC2::Device to be
constructed with a Hwc2::Composer (interface) pointer instead of a
service name. This means that now SurfaceFlinger itself constructs the
Hwc2::impl::Composer using the service name, rather than passing it down
a call chain.

Test: Code builds
Bug: None
Change-Id: Ic79f645cee40c534651b9c7b70f05497d98e51dc