Bug fix of bandwidth estimator overflow

Fix an overflow bug in bandwidth estimation filter. When the device is
up more than 25 days, there is a chance to hit overflow in time delta calculation. The chance is about 0.2% according to the metrics.

Bug: 238921804
Test: atest FrameworksTelephonyTests:com.android.internal.telephony.data.LinkBandwidthEstimatorTest
Change-Id: I36f6e7a9995af093a22bfb5d6e604448a1359acb