Don't allocate new typefaces when not needed.

The Typeface.create(Typeface, int) API can be used to create a new
typeface derived from an existing typeface. The implementation would
however create a new Typeface instance on every call, creating
unnecessary garbage in text-heavy applications. This changelist
adds a small cache of derived typefaces to avoid allocations whenever
possible.

Change-Id: Ib7290b3183e372aae12cf66033690e15cd4d0691