Initial pass on adding support for persistence using sqlcipher

To keep the database schema simple we just save the
otpauth:// url and let KeyInfo take care of checking for
validity.

This patch also includes multiple fixes for the KeyInfo
class.

We still need a separate activity to allow the user to
enter their PIN/password. Currently, "test" is used as the
password for the database.

app/src/main/java/me/impy/aegis/crypto/CryptoUtils.java

@@ -0,0 +1,16 @@
+package me.impy.aegis.crypto;
+
+import java.util.Arrays;
+
+public class CryptoUtils {
+    private CryptoUtils() {
+    }
+
+    public static void zero(char[] data) {
+        Arrays.fill(data, '\0');
+    }
+
+    public static void zero(byte[] data) {
+        Arrays.fill(data, (byte)0);
+    }
+}