LayoutLib: Misc rendering fixes.

- always set up the stroke. Paint may not have the proper
  style when drawing lines. stroke should still be setup.

- Fixed vertical linear gradient. Old code generated
  a gradient ratio of NaN

- Fixed alpha rendering when using shaders. In that
  case the alpha channel from the paint color should be
  used in conjunction with the shader.

- Fixed miter limit. Java expects the value to be multiplied
  by the stroke width

- Fixed support for drawing ALPHA_8 bitmaps. Java2D doesn't
  have bitmaps with only alpha channels, so we keep using
  ARGB bitmaps but when drawing them into a bitmap we erase
  the color information.

Change-Id: I4f04341fc843e3f7dadd1fdbf709b11a4f1e24b9