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.