Avoid thrashing the glyph cache during the precache phase

It is unlikely, but possible, to draw so many glyphs in a frame
(especially of the glyphs are quite large) that the cache starts flushing
itself to fit the later glyphs in. This causes unnecessary thrashing, because
when we actually draw the frame, we will again need to flush to fit the
earlier glyphs in, and then flush again to fit the later ones in.

It is better to avoid thrashing the cache at the precache phase, and wait
until we actually draw the glyphs that do not fit to do any eviction of
the earlier glyphs.

This change simply notes when we are in the preaching phase, and avoids flushing
the cache when a glyph does not fit.

Issue #7081725 avoid thrashing cache during DisplayList recording

Change-Id: I230410ab5b478091b1032fa99dc1752acf868bbe