Consider MMS as mutable cap

Previously, to accomodate wlan specifc APN, telephony dynamically add and remove MMS cap from the default network depending on QNS's preference.
However, since APN capabilities are considered immutable by connectivity, every dynamic change triggers an internet validation, which drains bettery power.
Terefore, in this change, telephony no longer recreate network agent, thus connectivity would no longer perform internet validation.
The consequence of this change is that connectivity won't perform network re-assign for the apps who already knows the old network, but it's fine because MMS requests are short-lived and only MMS service make such request.

Fix: 347018492
Test: daily check b/347018492#comment33
Test: send MMS w/ TMO using WLAN and cellular, reviewed log
Test: atest datanetworktest.java
Flag: NONE associated with a nextfood rolled out flag, time not allowed to rollout with another separate flag to deliver along with the original fix
Change-Id: I498d4195434d06c6c28f925ba76a1df6a6c218f4