Run derive_classpath in VM

This allows us to get an accurate list of APEXes that contribute to
the classpaths, including staged APEXes. (Note that derive_classpath
itself is in an APEX, and we will run it from a staged APEX if
appropriate.)

Add com.android.sdkext (which contains derive_classpath) as an
explicit dependency; also fixed formatting while I was at it.

Remove the old initializeClasspaths method as it is now redudant.

Tweaked build.prop values to allow derive_classpath to run; more work
is needed though (b/189164487).

Bug: 200808307
Test: composd_cmd test-compile
Change-Id: Iae677274ebac1cdf721c5497c1d82eb22d2277db
diff --git a/compos/apk/assets/vm_config.json b/compos/apk/assets/vm_config.json
index d008c12..0e97228 100644
--- a/compos/apk/assets/vm_config.json
+++ b/compos/apk/assets/vm_config.json
@@ -18,7 +18,10 @@
       "name": "com.android.compos"
     },
     {
+      "name": "com.android.sdkext"
+    },
+    {
       "name": "{CLASSPATH}"
     }
   ]
-}
\ No newline at end of file
+}