Use SecurityLevel integration on android

In !1522 (merged) we decided to delay using the android integration proposed in !1520 (merged).

This issue tracks:

Stop using android-only preferences for the security level. SecurityLevel.sys.mjs is the complete manager of these preferences.
Use the level name ("standard", "safer", "safest") instead of integers (4, 2, 1).
Read and write to SecurityLevel.sys.mjs via the EVENT_SECURITY_LEVEL_GET and EVENT_SECURITY_LEVEL_SET_BEFORE_RESTART.
In the latter case, wait for the event to resolve before we trigger the application restart to make our implementation more stable.

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