Clean up ADB over network

- Don't read or set "persist.service.adb.enable", it is deprecated
  and is read and cleared by UsbDeviceManager every boot. Trying
  to read it will always yield "" because it was already cleared
  by UsbDeviceManager (thus disabling adb debug every boot). Trying
  to set it doesn't have any effect because adb is now started by
  "sys.usb.config".

- In UsbDeviceManager, don't call AdbSettingsObserver when enabling
  or disabling adb over network because that function is only for
  enabling/disabling adb.

Change-Id: Ib6c1a655277c7f6d7395595ccfe3bac29c0582fe