Add temporary file storage as discussed in #665

Writable from OpenKeychain, readable worldwide. Should be used to write shared files to it by first creating the file using TemporaryStorageProvider.createFile and then write to the Uri returned.

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/DatabaseUtil.java

@@ -0,0 +1,36 @@
+package org.sufficientlysecure.keychain.util;
+
+import android.text.TextUtils;
+
+/**
+ * Shamelessly copied from android.database.DatabaseUtils
+ */
+public class DatabaseUtil {
+    /**
+     * Concatenates two SQL WHERE clauses, handling empty or null values.
+     */
+    public static String concatenateWhere(String a, String b) {
+        if (TextUtils.isEmpty(a)) {
+            return b;
+        }
+        if (TextUtils.isEmpty(b)) {
+            return a;
+        }
+
+        return "(" + a + ") AND (" + b + ")";
+    }
+
+    /**
+     * Appends one set of selection args to another. This is useful when adding a selection
+     * argument to a user provided set.
+     */
+    public static String[] appendSelectionArgs(String[] originalValues, String[] newValues) {
+        if (originalValues == null || originalValues.length == 0) {
+            return newValues;
+        }
+        String[] result = new String[originalValues.length + newValues.length ];
+        System.arraycopy(originalValues, 0, result, 0, originalValues.length);
+        System.arraycopy(newValues, 0, result, originalValues.length, newValues.length);
+        return result;
+    }
+}