Upgrade Android build to Gradle build system

The old Ant build system has been deprecated for a while and new development is focused on Gradle.
I also removed a hardcoded string that lint caught and moved the patch files to a subdirectory.
I left the JNI files in the root directory.

build/android/patches/libiconv_stdio.patch

@@ -0,0 +1,13 @@
+--- a/srclib/stdio.in.h	2011-08-07 15:42:06.000000000 +0200
++++ b/srclib/stdio.in.h	2015-06-10 09:27:58.129056262 +0200
+@@ -695,8 +696,9 @@ _GL_CXXALIASWARN (gets);
+ /* It is very rare that the developer ever has full control of stdin,
+    so any use of gets warrants an unconditional warning.  Assume it is
+    always declared, since it is required by C89.  */
+-_GL_WARN_ON_USE (gets, "gets is a security hole - use fgets instead");
++/*_GL_WARN_ON_USE (gets, "gets is a security hole - use fgets instead");*/
++#define gets(a) fgets( a, sizeof(*(a)), stdin)
+ #endif
+
+
+#if @GNULIB_OBSTACK_PRINTF@ || @GNULIB_OBSTACK_PRINTF_POSIX@