graphics: add a default implementation for IComposer

The default implementatoin is built on top of conventional hwcomposer2.

Test: make
Change-Id: Ia4f13b1d682462769056679fc0aae82a75cb53d2
diff --git a/Android.bp b/Android.bp
index d73f80b..dace522 100644
--- a/Android.bp
+++ b/Android.bp
@@ -8,6 +8,7 @@
     "graphics/allocator/2.0",
     "graphics/allocator/2.0/default",
     "graphics/composer/2.1",
+    "graphics/composer/2.1/default",
     "graphics/mapper/2.0",
     "graphics/mapper/2.0/default",
     "light/2.0",