Themes: Add palettized icon background support

This feature allows a theme to supply a single icon background,
usually a white or gray scale image, which will then be tinted using
a color from a palette of six color styles.

To use this new feature a theme designer will need to include the
new <paletteback /> tag which has the following syntax:

<paletteback
    img="name_of_png_drawable"
    swatchType=["vibrant" | "vibrantLight" | "vibrantDark" | "muted" | "mutedLight" | "mutedDark"]
    defaultSwatchColor1="hex_color"
    defaultSwatchColor2="hex_color"
    ⋮
    defaultSwatchColorN="hex_color"
    />

ex:
<paletteback
    img="iconback_palette"
    swatchType="vibrantDark"
    defaultSwatchColor1="#009688"
    defaultSwatchColor2="#cc0000"
    defaultSwatchColor3="#ff8800"
    />

You should specify some default swatch colors for those cases when
the original icon did not have any colors to match the swatchType
you requested. If no default swatch colors are defined, the original
un-tinted background will be used.

Change-Id: Ifd6dd65cc34ad4cd966fa9670f68704ac5671960