Improve performance when getting styled string.

The style used in the composing text for input methods
takes a long time to create. This is experienced as a
lag when composing the first word.

The bottleneck lies in the 10 calls to
nativeIndexOfString which does a linear search through
thousands of strings.

Change-Id: I3184b2be3673d384cca19e9a70ad94b4d3085576