Bug: 21588539 Move CHANGE_NETWORK_STATE to be in SYSTEM_SETTINGS

Merge the CHANGE_NETWORK_STATE permission with WRITE_SETTINGS.

AndroidManifest.xml:
Raised the protection level of CHANGE_NETWORK_STATE permission from
normal to signature|appops and pre23|preinstall for compatibility

provider/Settings:
Wrote new helper methods to check if app is allowed to change network
state.

ConnectivityManager.java & ConnectivityService.java:
Replace enforcement checks for CHANGE_NETWORK_STATE with
checkAndNoteChangeNetworkStateOperations instead.

Change-Id: If8c2dd3c76a5324ca43f1d90fa17973216c2bcc5