Switch to source-built GSON.

Bug: 243950468
Test: local build, presubmit
Change-Id: I3b800e30b99f52ba0f0fd1d0140cdbad611ba869
diff --git a/Android.bp b/Android.bp
index 36732ae..ee27cdd 100644
--- a/Android.bp
+++ b/Android.bp
@@ -17,8 +17,8 @@
     main_class: "com.android.manifmerger.Merger",
     static_libs: [
         "manifest-merger-jars",
-        "guava-21.0",
-        "gson-prebuilt-jar",
+        "guava",
+        "gson",
         "kotlin-stdlib",
     ],
 }