Fix lint warnings, don't require external storage permission for SDK >18

app/build.gradle

@@ -41,7 +41,6 @@ android {
         disable 'MissingTranslation'
         disable 'RtlEnabled'
         disable 'Typos'
-        disable 'UnusedAttribute'
     }
     packagingOptions {
         exclude 'LICENSE'

app/src/main/AndroidManifest.xml

@@ -20,13 +20,17 @@
     <uses-permission android:name="android.permission.WRITE_CALENDAR" />
     <uses-permission android:name="android.permission.READ_SYNC_SETTINGS" />
     <uses-permission android:name="android.permission.WRITE_SYNC_SETTINGS" />
-    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="18" />
+
+    <!-- for writing external log files; permission only required for SDK <= 18 because since then,
+    writing to app-private directory doesn't require extra permissions -->
+    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" android:maxSdkVersion="18" tools:ignore="UnusedAttribute"/>
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="18" tools:ignore="UnusedAttribute"/>
 
     <uses-permission android:name="org.dmfs.permission.READ_TASKS" />
     <uses-permission android:name="org.dmfs.permission.WRITE_TASKS" />
 
     <application
-        android:allowBackup="true" android:fullBackupContent="false"
+        android:allowBackup="true" android:fullBackupContent="false"  tools:ignore="UnusedAttribute"
         android:icon="@drawable/ic_launcher"
         android:label="@string/app_name"
         android:theme="@style/AppTheme">
@@ -110,7 +114,7 @@
         <activity
             android:name=".ui.settings.AccountActivity"
             android:label="@string/settings_title"
-            android:parentActivityName=".ui.settings.SettingsActivity" >
+            android:parentActivityName=".ui.settings.SettingsActivity" tools:ignore="UnusedAttribute">
         </activity>
 
         <!-- MemorizingTrustManager -->

app/src/main/java/at/bitfire/davdroid/resource/LocalTaskList.java

@@ -96,6 +96,7 @@ public class LocalTaskList extends AndroidTaskList implements LocalCollection {
 
 
     @Override
+    @SuppressWarnings("Recycle")
     public String getCTag() throws CalendarStorageException {
         try {
             @Cleanup Cursor cursor = provider.client.query(taskListSyncUri(), new String[] { COLUMN_CTAG }, null, null, null);