Refactor RuntimeShader to support easier assignment of uniforms.

uniforms are no longer set as single block of bytes where the caller
has to know the offset, but instead by the string identfier in the
shader.  This CL only supports floats, but future expansions of this
will provide helpers for ints, colors, and byte[].

Also by storing the shaders and uniforms in the RuntimeShaderBuilder
we can more easily copy them.  This enables Canvas.drawRipple to not
just copy the compiled SkSL effect, but also all the uniforms and
other input shaders set on the RuntimeShader.

Bug: 177051137
Test: HwAccelerationTest
Change-Id: I4733f42ba662546be6bebc37f0b89832778e66ce