Fix highlighting for phrase queries

I think the bug was introduced during a cleanup in Iaabc10c.
I don't think that " should be part of the legalSearchChars at query
time, it seems to break the regex.
The strategy here is to distinguish legalSearchChars used query time vs
the ones used at index time by introducing:
SearchEngine::legalSearchCharsForUpdate()

Bug: T167798
Change-Id: I61dc53665e26d3c6c48caed78dd3bbde9a33def7