Fix blending behavior with first draw op

Bug: 65077146
Test: Manual - uirendering tests don't allow test draw content
to be displayed first.

It's not always valid to disable blending on the first draw to the framebuffer,
since some blend modes affect the framebuffer in different ways. We now only
disable blending if the op is SRC_OVER to be safe.

For example:

    canvas.drawColor(0xfeff0000, PorterDuff.Mode.CLEAR);
    canvas.drawColor(Color.BLUE, PorterDuff.Mode.DST_OVER);

The BLUE should always be seen - the other draw should just clear the buffer.
Prior to this fix, the above code (put in a window background) would draw black.

In addition, this removes the disable behavior in drawRects(), since that should
never benefit from the optimization - that decoration is always drawn at the end
of a frame.

Change-Id: I34e8d9d62d6e1dfa00e9301f44c277475f2940a8