Report average error of bandwidth esitmation

Report average error instead of root mean square error of bandwidth
estimation. This simplifies the calculation and is also more suitable
for multiple-parameter estimator.

Updated-PDD: TRUE
Bug: 186643243
Test: atest com.android.server.wifi
Change-Id: Ie28b63ae5e35fd26c71a8a4dd18796567e6e9bfd