Use utf8 charset for searchindex MySQL table

This prevents the DB error "1283 Column 'si_title' cannot be part of
FULLTEXT index", which happens when the "binary" charset is used.

Note that 2ab7c9ec3b fixed the problem for creation of new DBs, though
not for reuse of existing DBs. It is still necessary to specify the
character set for the table itself.

Bug: 47191
Change-Id: Ib4f30fd2730839af1323295898e6dce47b27068d
Merge "Use utf8 charset for searchindex MySQL table"