Add new property to debug non-rectangular clip operations

This change adds a new property called "debug.hwui.show_stencil_clip"
that accepts the following values:

- "highlight", colorizes in green any drawing command that's tested
  against a non-rectangular clip region
- "region", shows the non-rectangular clip region in blue every time
  it is used
- "hide", default value, nothing is shown

Change-Id: I83c8602310edc4aaeb8b905371cdd185b17d32b5