Use activity context for Toast.makeText

[g-jiannan]

It is problematic to use application context for UI use case since Android 31. And it will cause StrictMode violation (see detectIncorrectContextUse).

[CLAassistant]

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[g-jiannan]

https://developer.android.com/guide/topics/ui/notifiers/toasts once recommended using Application or Activity but later changed to Activity only: https://r.android.com/2597393.

Since RequestPermissionActivity invokes finish() immediately after Toast.makeText, I'm not pretty sure if this PR is valid. Maybe it is inappropriate to use toast in this case.

[pyricau]

Thanks!