Bug fixes of LinkBandwidthEstimator

1)Add overflow prevention logic in link bandwidth estimator.
2)When the network is unknown, stop bandwidth update.
3)Invalidate sample in the beginning of post-poll processing.
4)Notify bandwidth esitmation to all dataconnections
5)Disable polling when it is not on active data phone.
6)Report average error of bandwidth estimation.

Bug: 186643243
Bug: 186178206
Test: atest FrameworksTelephonyTests:com.android.internal.telephony
Change-Id: I05349936b5aaffb43ee1024f61b30f73ad908cb0