[feature] Prevent lock bypass

[pikta-antis]

Hello there. I see App Lock Flutter is a promising app, and pretty much the only maintained open source alternative for this task. However, it's currently lacking some essential features, which makes the app very easy to bypass. Please consider adding:

• ability to protect the Flutter upon opening - it's very easy just to open the Flutter and uncheck the protection for any required app. In theory Flutter itself could be selected in the list of apps and be marked for protection, but this doesn't work - no password is still requested upon opening Flutter
• ability to set Flutter as a device admin app - it's easy to find Flutter in the list of background apps and stop it from running. Setting it as admin app would prevent it from being stopped
• uninstallation prevention - combine admin app feature with default protection of package manager, this should prevent the app being bypassed by uninstalling it

Other nice to have features could be:

• ability to unlock with patterns. Also, don't be lazy with 3x3 pattern boxes. At least my rom supports boxes up to 6x6 in size, which is much better