Support for fake bold in Minikin builds

Part of the fix for bug 15436379 Fake bold doesn't fully work (Minikin)

This patch queries the Minikin layout for when fake bold is needed,
and applies that to both shaping and drawing paint.

Also simplifies refcounting (the lifetime of all MinikinFont objects
is subsumed by the enclosing FontCollection).

Note: the fake bold flag set by the user is ignored in this patch.

Fake italics would be possible using the same mechanism, but it's
slightly more complicated (fake and user-set textSkewX values would need
to be combined, and the latter restored after drawing).

Change-Id: Ica2c4604846cbb37e5a783778b18d8993c9d4563