We used to have a registry which registered a single handlerwith RN.
THis was registered really early in the app.
When React Navigation was introduced we ddidn't realize it interacts
with the back button. In a stack nagigator it will navigate to the
previous screen. This meant our back button handling was broken.
This commit removes our previous registry and uses the RN back button
handler directly in the 2 components that use it: the conference and
bottom sheets.
Since these handlers are registered after navigation, our handlers are
going to run first so we cna implement the behavior we need, namely to
dismiss an open botom sheet or set the conference in PiP mode.
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