Change config_bandwidthEstimateSource to bandwidth_estimator

Change the default value of  config_bandwidthEstimateSource to
bandwidth_estimator.

Bug: 177942435
Bug: 176814680
Test: atest -c FrameworksTelephonyTests:com.android.internal.telephony
Test: manual test in 2G/3G/4G networks
Change-Id: I59c869334a6147879331c428baeb9625e0802f6e