shill: cellular: Fix fake MAC address for PPP dongles.

shill sets the 'Address' property of a device to the hex string
representation of the device's MAC address. This CL changes the code to
use the same format for PPP dongles (i.e. removing the colons in the
fake address 00:00:00:00:00:00).

BUG=chromium:249027
TEST=Tested the following:
1. Build and run unit tests.
2. Plug in a 3G dongle, and verify that the 'Address' property of the
   device associated with the dongle is "000000000000" instead of
   "00:00:00:00:00:00".

Change-Id: I0e5d110f8e788fd1a4b02c821b65f628a02c72c4
Reviewed-on: https://chromium-review.googlesource.com/168040
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/modem.cc b/modem.cc
index 6348324..e7387c4 100644
--- a/modem.cc
+++ b/modem.cc
@@ -27,7 +27,7 @@
 
 // statics
 const char Modem::kFakeDevNameFormat[] = "no_netdev_%zu";
-const char Modem::kFakeDevAddress[] = "00:00:00:00:00:00";
+const char Modem::kFakeDevAddress[] = "000000000000";
 const int Modem::kFakeDevInterfaceIndex = -1;
 size_t Modem::fake_dev_serial_ = 0;