[camera_android, camera_android_camerax] Protect Context registered BroadcastReceivers

[camsim99]

May have to split this into two. Fixes flutter/flutter#163818.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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[Piinks]

Greetings from stale PR triage! Is this one still on you radar @camsim99?

[camsim99]

@Piinks yes! hoping to get to it in the next week or two.