make SqliteDatabase ConflictAlgorithm and associated methods public.

SQLiteDatabase.java has ConflictAlgorithm which allows callers to specify
actions to take when insert or update causes constraint violations.
These actions are documented at http://www.sqlite.org/lang_conflict.html.

why make these public? usecase is the following:
Gmail has a table with a column "_id" being the integer primary key and
they let sqlite assign key values to the column.
but there is another UNIQUE key column (message_id) in the table.
so an insert could fail due to constraint violation on the message_id column
(i.e., not on the primary key). and when that happens, they would like to
get the value of _id that caused constraint violation.

currently hidden method insertOnConflict() already provides the above
functionality. that means exposing ConflictAlgorithm also.