Make it an error to use a bare apostrophe in aapt, and adjust warnings/errors.

In practice, no one ever writes an apostrophe in an aapt string with the
intent of using it to quote whitespace -- they always mean to include a
literal apostrophe in the string and then are surprised when they find
the apostrophe missing.  Make this an error so that it is discovered
right away instead of waiting until late in QA or after the strings have
already been sent for translation.  (And fix a recently-introduced string
that has exactly this problem.)

Silence the warning about an empty span in a string, since this seems to
annoy people instead of finding any real problems.

Make the error about having a translated string with no base string into
a warning, since this is a big pain when making changes to an application
that has already had some translations done, and the dead translations
should be removed by a later translation import anyway.