Add optional app lock behind biometric or device credential

Implements a full app lock (issue #1313) gating Aurora Store behind the
device biometric or, where unavailable, the screen-lock credential
(PIN/pattern/password) so it also works on TVs and older phones.

- AppLockManager: process-scoped, in-memory lock state so a cold start is
  always locked, with a short background grace timeout to avoid
  re-prompting during the install dialog or the biometric sheet itself.
- AppLockAuthenticator: BiometricPrompt wrapper using
  BIOMETRIC_STRONG | DEVICE_CREDENTIAL on API 30+ and
  setDeviceCredentialAllowed on older releases, plus an enrollment check.
- ComposeActivity (now a FragmentActivity) gates content via a three-state
  machine (AUTHENTICATING/LOCKED/UNLOCKED); the authenticating state shows
  a blank surface behind the prompt so dismissing it never flashes the
  lock card. Re-locks on return past the timeout and sets FLAG_SECURE
  while locked.
- Toggle lives on a dedicated Security preference screen, reached from
  Settings like the other preference entries.

app/src/main/res/drawable/ic_lock.xml

@@ -0,0 +1,24 @@
+<!--
+  ~ Copyright (C) 2026 The Android Open Source Project
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="960"
+    android:viewportHeight="960">
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+      android:fillColor="#e3e3e3"/>
+</vector>