Add a converter for classpaths.proto between proto and json/text formats.

The proto is used at build time to define classpath fragments to be read
and parsed at runtime by `derive_classpath` service; in order to define
and set BOOTCLASSPATH, DEX2OATCLASSPATH, and SYSTEMSERVERCLASSPATH
environ variables.

The tool is used for debugging and generation of binary protos.

Bug: 180105615
Test: m && launch_cvd with a follow up changes
Change-Id: I7e8b084e5f63e9aaad11da3221b909818e69e235

scripts/Android.bp

@@ -254,3 +254,22 @@ python_binary_host {
         "linker_config_proto",
     ],
 }
+
+python_binary_host {
+    name: "conv_classpaths_proto",
+    srcs: [
+        "conv_classpaths_proto.py",
+    ],
+    version: {
+        py2: {
+            enabled: false,
+        },
+        py3: {
+            enabled: true,
+            embedded_launcher: true,
+        },
+    },
+    libs: [
+        "classpaths_proto_python",
+    ],
+}