Ignore stats during network transition

Ignore stats for bandwidth estimation during network (PLMN/RAT) transition.
The traffic stats and modem activity report could be inaccurate
during that transition.

Bug: 188732673
Test: atest FrameworksTelephonyTests:com.android.internal.telephony
Change-Id: I4c8c652dc0c0897fceb4583e379dc4a1760170b7