Port wimax support changes .

- Register WiMax service in ContextImpl (fixes 4G toggle).
- Fix misdocumented EXTRA_4G_STATE in WimaxManagerConstants.
- Share wimaxClassLoader between WimaxHelper and ConnectivityService.
- Add ACCESS_4G_RADIO_DISABLE, ACCESS_4G_STATE, and CHANGE_4G_STATE
  permissions to SystemUI.

Note that WimaxManagerConstants was previously misdocumented, confabulating
the lookup key EXTRA_4G_STATE ("4g_state") with EXTRA_WIMAX_STATUS
("wimax_status"), which no longer exists.  This is also fixed.

Change-Id: I4f5973e7bd2ff4c860036df0f38589b34ad72b49