Use NetworkCallback to monitor connectivity changes on newer API levels

[fractalwrench]

Goal

We rely on the ConnectivityManager to detect whether or not we have a network connection. This allows us to save battery by avoiding unnecessary radio use when there is no available connection.

Several APIs in NetworkInfo (returned by ConnectivityManager) have been deprecated when targeting API 28 - namely isConnectedOrConnecting(), and getType() which sets the device.networkStatus field.

This changeset refactors the notifier to use modern APIs where possible, and otherwise falls back to the legacy behaviour on API < N.

This PR also adds Kotlin as a dependency to the bugsnag-android artefact.

Changeset

• Added Kotlin stdlib as a compile-time dependency
• Enabled allWarningsAsErrors for Kotlin compilation, which fails the build when using deprecated code
• Created a Connectivity interface that encapsulates all methods related to network state
• Created a ConnectivityCompat shim that forwards method calls onto the modern or legacy implementation, dependent on API level

Within the Connectivity implementations:

• Created a higher order function as a constructor parameter, which is invoked whenever the network connection changes
• Registered a BroadcastReceiver to listen to network changes on API < N, and a NetworkCallback otherwise
• Used networkInfo.type to get network state on API < N, and activeNetwork otherwise

Tests

Verified that reports are cached then automatically flushed when a device is put into airplane mode, on API 27 + API 22 physical devices, and that device.networkStatus is populated correctly.

Note: the unit tests currently fail locally due to many client objects being created. The fix for this is implemented in #503.

[bengourley]

Verified that reports are cached then automatically flushed when a device is put into airplane mode, on API 27 + API 22 physical devices, and that device.networkStatus is populated correctly.

I did this same manual check on API 29 and API 26.

Read through the code and mostly understood it. Looks sensible. Yay for Kotlin!