Add support for downgrading database version in SQLiteOpenHelper

Provide an API to allow application to support downgrade in databases
managed with SQLiteOpenHelper. Since onUpgrade() is now called only
if requested version is newer than current one, this adds support for
a similar onDowngrade() method, so customers can implement graceful
downgrading. If no downgrade method is implemented by caller, this
fallback to current behavior by throwing an exception.

Bug: 1609474
Change-Id: I0e0166984d4a700b1c43ce780b91244bc2fc87a6