liblog: logprint: deal with malformed log messages

(cherry picked from commit 083c5346)

Try to print as much content as possible should the application
logging only submit content as part of a tag with an empty message.
We search for the first non-printable ascii character in the tag, in
order to split it up for printing.

Applications (such as com.yahoo.mobile.client.android.weather) that
malform their log messages will no longer be punished by truncating
the content, but this should never be considered advocacy for their
bad behavior.

Bug: 27585978
Change-Id: Idb0680e8d6a6ad2bef5150661905acccb5b70afb