Pin jarjar targets to java_version 1.8.

This CL pins the following make targets to java_version 1.8,
which is currently the default:
  framework-protos
  repackaged.android.test.base
  repackaged.android.test.mock
  repackaged.android.test.runner

For consistency, their dependencies,
  android.test.base
  android.test.mock
  android.test.runner
which contain .java source files, are also pinned to 1.8.

This is so that the two steps:
 a) update jarjar to support v53 class files
 b) support -target 1.9 in the rest of the toolchain
can be completed in any order, in future CLs. Before this CL,
they would have needed to be completed in order a), b).

Bug: 72703434
Test: EXPERIMENTAL_USE_OPENJDK9=true USE_R8=true make checkbuild docs
      (in a client where CL http://r.android.com/596874 was reverted)

Change-Id: If78067294ae7ab78997aa109b0e08be427bdf0b8
diff --git a/proto/Android.bp b/proto/Android.bp
index 95f453c..f3811bd 100644
--- a/proto/Android.bp
+++ b/proto/Android.bp
@@ -6,6 +6,8 @@
     },
     srcs: ["src/**/*.proto"],
     no_framework_libs: true,
+    // Pin java_version until jarjar is certified to support later versions. http://b/72703434
+    java_version: "1.8",
     target: {
         android: {
             jarjar_rules: "jarjar-rules.txt",