[acm] Fix Network.preferred() -> Network.prefer()

[scott-brust]

Problem

1. Network.preferred() is less clear than simply Cellular.prefer()
2. Network.preferred() will both set the desired network back to default, and then return Network instead of the actual preferred network reference.

Solution

1. Network.preferred() renamed to be clearer ie: WiFi.prefer(true)
2. Network.prefer() will not reset the default network. The supported ways to clear any network preference are:
   a) Network.prefer(false) and b) explicitly calling .prefer(false) with the preferred interface, ie WiFi.prefer(false)

Steps to Test

Use connection-manager test application, item 2

Example App

connection-manager

References

ACM MVP API

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)