Update the Warning text about the visibility of device name.

After enabling DHCP hostname option, the device name will be carried
in the DHCP packets when connecting to a Wi-Fi network, which is also
visible for the DHCP server (upstream AP or hotspot). Update the warning
text correspondingly.

Bug: 174644890
Test: m
Test: manual test to verify the warning text has been updated when
      changing the device name.(Settings > About Phone > Device name).
Change-Id: I031a03deb8d20fb3b3a217b0fa7d45aa1b3330d1