Add EffectiveConnectionType enum to the system profile proto

Value of the EffectiveConnectionType enum is provided by the
NetworkMetricsProvider.

A new class EffectiveConnectionTypeObserver has been added which listens
to the changes in the EffectiveConnectionType, and lives on the same
thread as the NetworkQualityEstimator.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

BUG=677158

Review-Url: https://codereview.chromium.org/2605553002
Cr-Commit-Position: refs/heads/master@{#442821}