Mail: Add visibility for some methods without method visibility

Mail: Add visibility for some methods without method visibility

Carefully, I checked usage of these methods in our code bases and
added the best possible visibilities to them. Not sure if I missed
something but let me know if I did.

Used private & public where suitable for the various methods. As for
__toString(), this is a magic method, so should be public per PHP docs.

Change-Id: Ie0987f4a984cac2f5eb1d9e21a305ad9467a8eb2