Call ProcessStrings() in ImageWriter, fix prefix check.

ProcessStrings was missing due to bad conflict resolution
in cherry-pick
    https://android-review.googlesource.com/114971

The prefix check was broken by the "cherry-pick"
    https://android-review.googlesource.com/115445
where in addition to a simple cherry-pick, an additional
comment was addressed and an error slipped in.

Change-Id: I130d1d1a2995fd4a4c27c918c2a0dc38724ce68f