Bugzilla

Following an [investigation](https://docs.google.com/document/d/1odxXh8lEPU7M1LMoBk79aViV3C0t7kc3hbml7qy8IcY/edit?usp=sharing) and various discussions, we have decided on a way forward for presenting the fullscreen notification.
The plan is:
- Modify the message used from "Entering full screen mode" to give more helpful instructions on how to exit fullscreen.
- To present the message, use a `Toast` instead of a custom `Dialog`.

Notes:
- Devices running pre-v12 versions of Android may require OS level security patches to protect them from 'toastburst' style attacks (see https://developer.android.com/privacy-and-security/risks/tapjacking#risk_custom_toast). 
- We should retest bugs associated with fullscreen. Toasts appear at a higher z-index than Dialogs, so we hope this change will reduce the likelihood of the fullscreen message being obscured by other prompts, but there may be situations where it is still preferable to exit fullscreen in response to some system event.

Following an [investigation](https://docs.google.com/document/d/1odxXh8lEPU7M1LMoBk79aViV3C0t7kc3hbml7qy8IcY/edit?usp=sharing) and various discussions, we have decided on a way forward for presenting the fullscreen notification.
The plan is:
- Modify the message used from "Entering full screen mode" to give more helpful instructions on how to exit fullscreen.
- To present the message, use a `Toast` instead of a custom `Dialog`.

Notes:
- Devices running pre-v12 versions of Android may require OS level security patches to protect them from 'toastburst' style attacks (see https://developer.android.com/privacy-and-security/risks/tapjacking#risk_custom_toast). (Previously, we moved away from using `Toast` for this reason, but plenty of time has now elapsed for manufacturers to offer security patches and users to apply them.)
- We should retest bugs associated with fullscreen. Toasts appear at a higher z-index than Dialogs, so we hope this change will reduce the likelihood of the fullscreen message being obscured by other prompts, but there may be situations where it is still preferable to exit fullscreen in response to some system event.