For each sdk built generate a JSON file describing its contents

Some build scripts need to know information about the contents of an
sdk, such as what APIs it provides (via java_sdk_library). Rather than
duplicate that information in the scripts or attempt to access that
information (where available) by looking at the contents of the
snapshot this change generates a JSON file that sits alongside the
snapshot itself.

The info file can be generated without generating the snapshot zip file
but whenever a snapshot zip file is generated the info is generated
too. The info file sits alongside the zip file in out/mainline-sdks.

Bug: 204763318
Test: m art-module-sdk
      m dist
Change-Id: I289530bb21693dc6443826c24c17c9b5d85d2d8b

sdk/java_sdk_test.go

@@ -795,6 +795,44 @@ sdk_snapshot {
 .intermediates/myjavalib.stubs/android_common/javac/myjavalib.stubs.jar -> sdk_library/public/myjavalib-stubs.jar
 .intermediates/myjavalib.stubs.source/android_common/metalava/myjavalib.stubs.source_api.txt -> sdk_library/public/myjavalib.txt
 .intermediates/myjavalib.stubs.source/android_common/metalava/myjavalib.stubs.source_removed.txt -> sdk_library/public/myjavalib-removed.txt
+`),
+		checkInfoContents(`
+[
+  {
+    "@type": "sdk",
+    "@name": "mysdk",
+    "java_header_libs": [
+      "exported-system-module",
+      "system-module"
+    ],
+    "java_sdk_libs": [
+      "myjavalib"
+    ],
+    "java_system_modules": [
+      "my-system-modules"
+    ]
+  },
+  {
+    "@type": "java_library",
+    "@name": "exported-system-module"
+  },
+  {
+    "@type": "java_system_modules",
+    "@name": "my-system-modules",
+    "@deps": [
+      "exported-system-module",
+      "system-module"
+    ]
+  },
+  {
+    "@type": "java_sdk_library",
+    "@name": "myjavalib"
+  },
+  {
+    "@type": "java_library",
+    "@name": "system-module"
+  }
+]
 `),
 	)
 }