shill: Switch T-Mobile from IPv6 only to IPv4v6 (dual stack)

On Qualcomm and Fibocom  devices crostini does not work T-Mobile IPv6 only network,
check https://groups.google.com/a/google.com/g/cros-networking-bugs/c/mpB5YOeTi8Q
and b/162701578. On Fibocom we are unable to connect to IPv6 only network as we get
a Link-local IPv6 address and IPv4 static address, as of now modemmanager ignores ipv4
address in ipv6 only network, causing shill to go in a loop.
So temporarily switching to dual stack for T-Mobile.

BUG=b:172696688,b:162701578, b:183029202

TEST=use T-Mobile IPv4v6 dual stack on trogdor, drallion; run
  cellular_Identifiers.tmobile, cellular_Smoke.tmobile

Change-Id: I05ef48a5dd8f771b4c32c3e1a810098653bec04e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2807245
Commit-Queue: Madhav . <madhavadas@google.com>
Tested-by: Madhav . <madhavadas@google.com>
Reviewed-by: Hugo Benichi <hugobenichi@google.com>
Reviewed-by: Taoyu Li <taoyl@chromium.org>
(cherry picked from commit e2c31be2c6dea118b5f77bc9ec1f4f470abdf6e2)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2995317
Reviewed-by: Madhav . <madhavadas@google.com>
Commit-Queue: Hugo Benichi <hugobenichi@google.com>
Tested-by: Hugo Benichi <hugobenichi@google.com>
(cherry picked from commit caf95f7930c0c7fe54a11cb5b868b2aa40a88b89)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/3001188
diff --git a/shill/mobile_operator_db/serviceproviders.prototxt b/shill/mobile_operator_db/serviceproviders.prototxt
index 6fb9922..74339a6 100644
--- a/shill/mobile_operator_db/serviceproviders.prototxt
+++ b/shill/mobile_operator_db/serviceproviders.prototxt
@@ -14016,7 +14016,7 @@
       localized_name {
         name : "LTE"
       }
-      ip_type: IPV6
+      ip_type: IPV4V6
     }
     mobile_apn {
       apn : "epc.tmobile.com"