Add support for new bandwidthConstrainedOk flag used to deliver notifications to satellite networks

[andrew-signal]

Discussion

Hi! The Android Developers page for Developing for constrained satellite networks indicates that we need to pass a new flag, bandwidth_constrained_ok in our AndroidConfig options, like so, to ensure delivery of our pushes on constrained satellite networks.

{
  "message":{
    "token":"bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...",
    "notification":{
      "title":"Portugal vs. Denmark",
      "body":"great match!"
    }
    "android": {
       "bandwidth_constrained_ok": true
    }
  }
}

In order to do this in a server app based on the firebase-admin-java SDK, we need to expose a config option in AndroidConfigBuilder.

(In practice, things seem to be okay without this flag, but the documentation says we should add it, so I'm inclined to add it.)

Testing

• I added a new unit test to match the already extant one for direct_boot_ok.
• mvn test passes locally on my machine.
• I expect CI will also run on this PR.

API Changes

• This does add the minimal new config parameter to set this parameter in AndroidConfigBuilder, which is the least intrusive change I can make to the public API to expose this now-needed functionality.

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[jonathanedey]

LGTM, Thank you @andrew-signal for the contribution!

[jonathanedey]

cc: @lahirumaramba, I'll merge this PR as is and have the docs updated and reviewed in a follow up pr along with the RestrictedSatelliteOk api.