Make default java_toolchain compile protos as java 7.

Fixes https://github.com/bazelbuild/bazel/issues/2829

Without this change, there is no way to use Bazel to build a release Android build (--noincremental_dexing) with java 8 and protos.

RELNOTES: Java protos are compiled to Java 7 bytecode.
PiperOrigin-RevId: 165500316

tools/jdk/BUILD

diff --git a/tools/jdk/BUILD b/tools/jdk/BUILD
index 8ed90ef..9029782 100644
--- a/tools/jdk/BUILD
+++ b/tools/jdk/BUILD
@@ -155,6 +155,16 @@
 java_toolchain(
     name = "toolchain",
     bootclasspath = [":bootclasspath"],
+    compatible_javacopts = {
+        # Restrict protos to Java 7 so that they are compatible with Android.
+        "proto": [
+            "-source",
+            "7",
+            "-target",
+            "7",
+            "-XDallowBetterNullChecks=false",
+        ],
+    },
     encoding = "UTF-8",
     extclasspath = [":extclasspath"],
     forcibly_disable_header_compilation = 0,