Don't overwrite link bandwidth values on getNetworkCapabilities

Values saved locally in DataConection and updated only when carrier
config changed (from DcTracker) or LCE update from modem.
getNetworkCapabilities() uses locally saved values instead of
overwriting every time.
If modem value is INVALID, use carrier config value instead.

Test: atest DataConnectionTest, DcTrackerTest
Test: manually verify updateLinkBandwidth called with NR_NSA,
NR_NSA_MMWAVE when ServiceState changes
Bug: 149864873
Bug: 150034051

Change-Id: Idb03c4f6bb6b9939024f13266f963be1f9b098b9
Merged-In: Idb03c4f6bb6b9939024f13266f963be1f9b098b9