Only use encrypted storage for Nightly populations

[ekager]

There are a small but not insignificant number of crashes accessing the secure preferences with the Keystore.

See examples:
#7043
#7044
#7045
#7046

In order to stabilize for release, we want to continue to monitor these crashes in Nightly, but use the plaintext storage for Release. We will have to add a way to do this in Fenix the same way that FxA state has an option for using encrypted/plaintext (see #6704 )

┆Issue is synchronized with this Jira Task

[ekager]

AC work here to add the flags: mozilla-mobile/android-components#5902
And then will require some Fenix work as well to set the flags

[ekager]

Closing because this is done