Bug 5250788: TextView gets slower as the text length grows

getSpans was called too many times in handleRun. Pre-compute the
subset of intersected spans and iterate over a subset of it instead.

Moving the instanceof test in getSpans after the other tests also
speeds things up a lot.

On a text with ~300 words, all with a span attached, getSpans went down
from 78% to 14% of the CPU usage.

Change-Id: I59bc44f610e9a548e0dcec68b180934da9e5c559