BiometricAuthenticator: implement biometric authentication for sensitive operations

When biometric hardware is available, it will be used to authenticate
the user before private keys are shown on screen or when zip exports
are executed.

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

ui/src/main/java/com/wireguard/android/fragment/TunnelEditorFragment.kt

@@ -25,6 +25,7 @@ import com.wireguard.android.backend.Tunnel
 import com.wireguard.android.databinding.TunnelEditorFragmentBinding
 import com.wireguard.android.fragment.AppListDialogFragment.AppExclusionListener
 import com.wireguard.android.model.ObservableTunnel
+import com.wireguard.android.util.BiometricAuthenticator
 import com.wireguard.android.util.ErrorMessages
 import com.wireguard.android.viewmodel.ConfigProxy
 import com.wireguard.android.widget.EdgeToEdge.setUpRoot
@@ -233,13 +234,27 @@ class TunnelEditorFragment : BaseFragment(), AppExclusionListener {
         if (!isFocused) return
         val edit = view as? EditText ?: return
         if (!haveShownKeys && edit.text.isNotEmpty()) {
-            if (true /* TODO: do biometric auth prompt */) {
-                haveShownKeys = true
-            } else {
-                /* Unauthorized, so return and don't change visibility. */
-                return
+            BiometricAuthenticator.authenticate(R.string.biometric_prompt_private_key_title, requireActivity()) {
+                when (it) {
+                    is BiometricAuthenticator.Result.Success, is BiometricAuthenticator.Result.HardwareUnavailableOrDisabled -> {
+                        haveShownKeys = true
+                        showPrivateKey(edit)
+                    }
+                    is BiometricAuthenticator.Result.Failure -> {
+                        Snackbar.make(
+                                binding!!.mainContainer,
+                                it.message,
+                                Snackbar.LENGTH_SHORT
+                        ).show()
+                    }
+                }
             }
+        } else {
+            showPrivateKey(edit)
         }
+    }
+
+    private fun showPrivateKey(edit: EditText) {
         requireActivity().window.addFlags(WindowManager.LayoutParams.FLAG_SECURE)
         edit.inputType = InputType.TYPE_TEXT_FLAG_NO_SUGGESTIONS or InputType.TYPE_TEXT_VARIATION_VISIBLE_PASSWORD
     }