graphics: move ComposerBase to HAL support library

Create android.hardware.graphics.composer@2.1-hal and add
ComposerHal, which is heavily based on ComposerBase, to it.

There are two bigger TODOs.  One is to remove the concept of
"clients" from the class and the other is to remove hwcomposer2.h
dependency.

Test: boots and VTS
Change-Id: I37b4fb3ae2239bf11aa87a56d1e2ebfe0b8c6b54