Add option in filter settings to show froxzen (disabled) apps

[nmarteybotchway]

Closes #433

Summary

Adds a new "Show frozen apps" filter checkbox to the main app list,
allowing users to view and configure firewall rules for disabled/frozen
apps without needing to unfreeze them first.

Problem

Previously, disabled (frozen) apps were completely hidden from
TrackerControl's app list. This made it impossible to:

• Pre-configure firewall rules before unfreezing an app
• Verify rules are in place for frozen apps
• Migrate rules from another firewall without temporarily unfreezing
  every disabled app

Changes

• res/menu/main.xml: Added menu_app_frozen checkbox item to the
  filter submenu, below "Show apps without internet"
• res/values/strings.xml: Added menu_show_frozen string resource
• ActivityMain.java onPrepareOptionsMenu: Added checked state for
  the new checkbox
• ActivityMain.java onOptionsItemSelected: Added handler that writes
  show_frozen to SharedPreferences when toggled
• ActivityMain.java onSharedPreferenceChanged: Added show_frozen
  to the preference change listener so the list refreshes automatically
  when toggled
• Rule.java getRules(): Added show_frozen preference read and
  updated the filter condition from rule.enabled to
  (show_frozen || rule.enabled) so disabled apps are included when
  the option is on

Behaviour

• Toggle OFF (default): frozen/disabled apps hidden, existing behaviour
  preserved
• Toggle ON: disabled apps appear in the list alongside active apps,
  fully configurable

Testing

• Disabled a system app via Settings → Apps → Disable
• Verified app was hidden with toggle off
• Verified app appeared in list with toggle on
• Verified firewall rules can be set on the disabled app
• Verified preference persists across app restarts
• Verified no visible change when no disabled apps are present on device

[nmarteybotchway]

Hey @kasnder - another small change here. Would really appreciate your thoughts when you get a chance.