Add Settings.Global.ADB_WIFI_ENABLED.

ADB_ENABLED historically meant the state for USB debugging. Since
wireless debugging can be enabled separately, define another setting
for it.

BUG: b/111434128

Test: make
Exempt-From-Owner-Approval: approved in aosp_master
Change-Id: If3abca8e77381d6832f55d55a43c52ee1a1267d1