Update system icon resolution in LayoutLib.

This fixes some bugs with the resolution of icons for Status Bar and
Navigation Bar by separating the icons into v21 and v11 directories. An
icon is searched in v21 directory by default. For simulating older
platforms, v11 directory is searched first and then v21 is used as a
fallback.

This also adds xxhdpi versions of home, back and recent icons.

Change-Id: Ia6eafc83856eacafc4fb0b13d87793d4aa24f106