Don't override dialog icon drawable unless valid resource ID was set

Fixes a regression where Builder.setIcon(Drawable) would get overridden
even when Builder.setIcon(int) had never been called and was still 0.
Fixes attribute id to respect all valid resource IDs (e.g. non-zero).
Updates documentation to reflect the long-standing override behavior.

BUG: 18904762
Change-Id: I905703993a59910555d5a858e0aaecab63221a02