Changes in framework for WIFI_CREDENTIAL_CHANGE broadcast. [DO NOT MERGE]

* Only allow system app to send/receive
  WIFI_CREDENTIAL_CHANGE.
* Define constants in WifiManager.

Bug: 15703353
Change-Id: I4bcc6bc82ec01952500eef3965cf81116bba693d
(cherry picked from commit f4a43f196e1132968a8add60efa4ae4558f8b010)
2 files changed