Define a unified protobuf message for "aapt2 optimize" metadata.

This helps solve three problems:

* obfuscation maps are now documented.
* obfuscation maps will not require hand-rolled parsers.
* no additional plumbing will be required for future
  optimizations---plumbing individual mapping files in Bazel for
  ProGuard, aapt2, etc. has been a pain.

Bug: 153201806
Test: make aapt2
Change-Id: Ie3829834527e8875d768ae28242d3aeca28c6db2
diff --git a/tools/aapt2/Android.bp b/tools/aapt2/Android.bp
index 7efe3c3..a4199b6 100644
--- a/tools/aapt2/Android.bp
+++ b/tools/aapt2/Android.bp
@@ -161,6 +161,7 @@
         "ApkInfo.proto",
         "Configuration.proto",
         "Resources.proto",
+        "ResourceMetadata.proto",
         "ResourcesInternal.proto",
         "ValueTransformer.cpp",
     ],
@@ -218,6 +219,7 @@
     srcs: [
         "Configuration.proto",
         "ResourcesInternal.proto",
+        "ResourceMetadata.proto",
         "Resources.proto",
     ],
     out: ["aapt2-protos.zip"],