[camera] Fix crash from changing permissions

[mklim]

Description

Our listeners are called for every single permissions change, not just
the first one. This resulted in multiple replies being sent to Dart and
fatal crashes. Change to instead manually keep track of responses and
only respond to the first call.

Related Issues

Fixes flutter/flutter#26228

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Breaking Change

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[bparrishMines]

LGTM

[mklim]

Thanks!