[DO NOT MERGE] Update CellIdentityGsm Constructor

Add a dummy parameter to the CellIdentityGsm constructor
for backwards compatibility.

As this code doesn't exist in goog/master, add a shim but
don't merge it.

Bug: 135921133
Test: compilation
Change-Id: I707bb4d2b90bfecd5825c25f9a1f92dbd9ccf41b