Fix a crash where user data was not validated.

ExtractedText#partialStartOffset and #partialEndOffset are
from the app, that sets it as it sees fit. We need to
validate them so that we don't crash.
Still emit a warning if this is the case, as this is
not expected.

Bug: 9570771
Change-Id: Id9d6babd1620da39bf0e454b14d7ce716bd9d9d3