Fix some lint errors and warnings:

- check the calling Intent action when autostarting
- replace the FAB + image with a vector drawable

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/externalevents/AutoStartReceiver.java

@@ -28,7 +28,7 @@ public class AutoStartReceiver extends BroadcastReceiver {
 
     @Override
     public void onReceive(Context context, Intent intent) {
-        if (GBApplication.getGBPrefs().getAutoStart()) {
+        if (GBApplication.getGBPrefs().getAutoStart() && Intent.ACTION_BOOT_COMPLETED.equals(intent.getAction())) {
             Log.i(TAG, "Boot completed, starting Gadgetbridge");
             GBApplication.deviceService().start();
         }

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

@@ -0,0 +1,9 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24.0"
+    android:viewportHeight="24.0">
+    <path
+        android:fillColor="#FFFFFFFF"
+        android:pathData="M19,13h-6v6h-2v-6H5v-2h6V5h2v6h6v2z" />
+</vector>

app/src/main/res/layout/activity_controlcenterv2_app_bar_main.xml

@@ -30,7 +30,7 @@
         android:layout_alignParentBottom="true"
         android:layout_alignParentEnd="true"
         android:layout_gravity="bottom|end"
-        android:src="@drawable/ic_add_white"
+        app:srcCompat="@drawable/ic_add"
         app:elevation="6dp"
         app:pressedTranslationZ="12dp"
         android:layout_marginBottom="30dp"

app/src/main/res/layout/activity_fragmentappmanager.xml

@@ -33,7 +33,7 @@
         android:layout_alignParentBottom="true"
         android:layout_alignParentEnd="true"
         android:layout_gravity="bottom|end"
-        android:src="@drawable/ic_add_white"
+        app:srcCompat="@drawable/ic_add"
         app:elevation="6dp"
         app:pressedTranslationZ="12dp"
         android:layout_marginBottom="30dp"