More updates in link bandwidth estimator

1) Add per carrier, TAC, RAT and signal level grouping.
2) Add smoothing filter.
3) Pass the BW update to default data connection when there is
significant BW change while BW source is bandwidth_estimator.
4) Add threshold values for low BW 2G and 3G modes.
5) Add dump() to dump internal states and local logs.

Bug: 177942435
Bug: 176814680
Test: atest -c FrameworksTelephonyTests:com.android.internal.telephony
Test: manual test in 2G/3G/4G networks
Change-Id: Ifc346cd445f43b368e5d825c8baf0f38eb9a7f10