updater: boot the installed slot via a configurable reboot reason

Replace the config_use_tryboot boolean with a config_abRebootReason string
passed straight to PowerManager.reboot() (empty = normal reboot). RPi
overrides it to "2" so the candidate slot boots via a one-shot reboot,2
(partition 2 = boot_b); tryboot is now reserved for recovery entry. No
behavior change on devices that leave the reason empty.

app/src/main/java/me/pawlet/updater/UpdaterReceiver.java

@@ -78,7 +78,7 @@ public class UpdaterReceiver extends BroadcastReceiver {
     public void onReceive(Context context, Intent intent) {
         if (ACTION_INSTALL_REBOOT.equals(intent.getAction())) {
             PowerManager pm = context.getSystemService(PowerManager.class);
-            pm.reboot(Utils.usesTryboot(context) ? "tryboot" : null);
+            pm.reboot(Utils.getAbRebootReason(context));
         } else if (Intent.ACTION_BOOT_COMPLETED.equals(intent.getAction())) {
             SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(context);
             pref.edit().remove(Constants.PREF_NEEDS_REBOOT_ID).apply();

app/src/main/java/me/pawlet/updater/UpdatesListAdapter.java

@@ -412,7 +412,7 @@ public class UpdatesListAdapter extends RecyclerView.Adapter<UpdatesListAdapter.
                 button.setEnabled(enabled);
                 clickListener = enabled ? view -> {
                     PowerManager pm = mActivity.getSystemService(PowerManager.class);
-                    pm.reboot(Utils.usesTryboot(mActivity) ? "tryboot" : null);
+                    pm.reboot(Utils.getAbRebootReason(mActivity));
                 } : null;
             }
             break;

app/src/main/java/me/pawlet/updater/misc/Utils.java

@@ -420,7 +420,14 @@ public class Utils {
         return new File(Constants.UPDATE_RECOVERY_EXEC).exists();
     }
 
-    public static boolean usesTryboot(Context context) {
-        return context.getResources().getBoolean(R.bool.config_use_tryboot);
+    /**
+     * Reason to pass to {@link android.os.PowerManager#reboot} when booting the
+     * freshly-installed slot, or {@code null} for a normal reboot. Devices whose
+     * firmware needs an explicit slot boot set config_abRebootReason (e.g. RPi
+     * uses "2" to one-shot boot partition 2 = boot_b).
+     */
+    public static String getAbRebootReason(Context context) {
+        String reason = context.getResources().getString(R.string.config_abRebootReason);
+        return (reason == null || reason.isEmpty()) ? null : reason;
     }
 }

app/src/main/res/values/config.xml

@@ -5,5 +5,9 @@
 -->
 <resources>
     <bool name="config_hideRecoveryUpdate">false</bool>
-    <bool name="config_use_tryboot">false</bool>
+    <!-- Reason passed to PowerManager.reboot() to boot the freshly-installed
+         slot. Empty = normal reboot. Devices whose firmware needs an explicit
+         slot boot override this (e.g. RPi sets "2" to one-shot boot partition 2
+         = boot_b); see the device vendor.prop / config.txt. -->
+    <string name="config_abRebootReason" translatable="false"></string>
 </resources>