WifiInfo: Documentation for change of SSID formatting

The getSSID() method used to return a string with no quotes before
Android J, so add documentation mentioning when this was changed.

Bug: 12957620
Test: Compiles & verified that the docs are updated (make
doc-comment-check-docs).
Change-Id: I5d3713303c8f8c5c09d74ad1c5b8398107847b87