SDK / exoplayer

In order to detect 5G-NSA, we need to remove Util.getNetworkType
and replace it with a class that is actively listening to changes.
The network type observation in DefaultBandwidthMeter already provides
the right framework to integrate this and in order to facilitate
further changes we move it to a separate Util class.

The overall effect of this change should be a complete no-op.

PiperOrigin-RevId: 363384567