Cleanup of Bitmap.h entry points.

All Bitmap constructors have been made private and the only way to
create one is through the allocate or createFrom factories.

SkColorSpace is now explicitly passed in to all the factories and is
no longer assumed to be sRGB.

Test: atest CtsGraphicsTestCases
Change-Id: I92c1c5c59df6de7fdd90e9504a2c2717cb854588