For #8324 - Only use encrypted storage for Nightly/Debug populations.

[ekager]

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[pocmo]

What will happen to release users that have been using this before? 🤔

[ekager]

All the features using secure storage were only enabled in Nightly (just Logins).
I have also added a check that attempts to migrate data from the other store (but won't crash if we can't access the keystore)
mozilla-mobile/android-components#5902