wifi: Formalize Settings.Global flag usages

a) Removed some of the @hide settings keys which can be replaced with
an overlay flag (i.e they are read only).
b) Promoted all the other wifi setting keys (which are read/write)
to @SystemApi.

Bug: 147373460
Test: Device boots up and connects to wifi networks.
Test: atest com.android.server.wifi
Change-Id: I836f440b66387bd8d92832f9c1a027d3ee70c6d8
16 files changed