Introduce indirection for base micro themes.

This is needed, since overlays must add/modify items to base themes, but
simply declaring the style in an overlay will override the parent. To
ensure the true parent is never overridden, a base theme is introduced
that both the regular theme and the overlay can inherit from.

Bug: 21425951
Change-Id: I0967d0b4ddf12e9765bc321a7485a6dbdfef0b1e