Improve performance of parsing unknown fields in Java (#9371)

Credit should go to @elharo for most of these Java changes--I am just
cherry-picking them from our internal codebase. The one thing I did
change was to give the UTF-8 validation tests their own Bazel test
target. This makes it possible to give the other tests a shorter
timeout, which is important for UnknownFieldSetPerformanceTest in
particular.

Makefile.am

@@ -493,6 +493,7 @@ java_EXTRA_DIST=
   java/core/src/test/java/com/google/protobuf/TypeRegistryTest.java                \
   java/core/src/test/java/com/google/protobuf/UnknownEnumValueTest.java            \
   java/core/src/test/java/com/google/protobuf/UnknownFieldSetTest.java             \
+  java/core/src/test/java/com/google/protobuf/UnknownFieldSetPerformanceTest.java             \
   java/core/src/test/java/com/google/protobuf/UnmodifiableLazyStringListTest.java  \
   java/core/src/test/java/com/google/protobuf/Utf8Test.java                        \
   java/core/src/test/java/com/google/protobuf/Utf8Utils.java                       \

java/core/BUILD

@@ -224,6 +224,7 @@ test_suite(
         "conformance_test",
         "core_build_test",
         "core_tests",
+        "utf8_tests",
     ],
 )
 
@@ -243,10 +244,12 @@ conformance_test(
 
 junit_tests(
     name = "core_tests",
-    size = "large",
+    size = "small",
     srcs = glob(
         ["src/test/java/**/*.java"],
         exclude = [
+            "src/test/java/com/google/protobuf/DecodeUtf8Test.java",
+            "src/test/java/com/google/protobuf/IsValidUtf8Test.java",
             "src/test/java/com/google/protobuf/TestUtil.java",
             "src/test/java/com/google/protobuf/TestUtilLite.java",
         ],
@@ -264,6 +267,24 @@ junit_tests(
     ],
 )
 
+# The UTF-8 validation tests are much slower than the other tests, so they get
+# their own test target with a longer timeout.
+junit_tests(
+    name = "utf8_tests",
+    size = "large",
+    srcs = [
+        "src/test/java/com/google/protobuf/DecodeUtf8Test.java",
+        "src/test/java/com/google/protobuf/IsValidUtf8Test.java",
+        "src/test/java/com/google/protobuf/IsValidUtf8TestUtil.java",
+    ],
+    deps = [
+        ":core",
+        "@maven//:com_google_guava_guava",
+        "@maven//:com_google_truth_truth",
+        "@maven//:junit_junit",
+    ],
+)
+
 java_lite_proto_library(
     name = "generic_test_protos_java_proto_lite",
     visibility = [
@@ -346,6 +367,7 @@ LITE_TEST_EXCLUSIONS = [
     "src/test/java/com/google/protobuf/TypeRegistryTest.java",
     "src/test/java/com/google/protobuf/UnknownEnumValueTest.java",
     "src/test/java/com/google/protobuf/UnknownFieldSetLiteTest.java",
+    "src/test/java/com/google/protobuf/UnknownFieldSetPerformanceTest.java",
     "src/test/java/com/google/protobuf/UnknownFieldSetTest.java",
     "src/test/java/com/google/protobuf/WellKnownTypesTest.java",
     "src/test/java/com/google/protobuf/WireFormatTest.java",