Make libhwui entirely optional.

The makefile variable USE_OPENGL_RENDERER must be set to true to compile
libhwui and the related code in the JNI layer.

This change also removes obsolete APIs from Canvas that must not be used
and would be confusing if left in. These APIs were remnants of our first
attempt at an OpenGL renderer for the view hierarchy and had not been
taken out before Android 1.0 was released.

Change-Id: I2475ff1307212bab26c926724f3c508681c7dae1