Switch to spongycastle

https://github.com/rtyley/spongycastle

It replaces bouncycastle2. Looks like a cleaner and better integration. A
precompiled .jar ist in the "lib/"-dir.

project.properties

@@ -0,0 +1,11 @@
+# This file is automatically generated by Android Tools.
+# Do not modify this file -- YOUR CHANGES WILL BE ERASED!
+#
+# This file must be checked in Version Control Systems.
+#
+# To customize properties used by the Ant build system use,
+# "ant.properties", and override values to adapt the script to your
+# project structure.
+
+# Project target.
+target=android-10