Finer grained character boundaries in computing SMS fragment lengths

The standard Java character iterator has potentially unbounded
distance between character boundaries, meaning that when
breaking an SMS message into fragments it's not safe to assume that
the fragment will contain such a boundary. This patch
special-cases flags (pairs of Regional Indicator Symbols) and also
guarantees some progress in the case of no boundary found.

Change-Id: I35990b44ad72887e1fdd436223808e18b04bd578
CRs-Fixed: 994916