core: Rewrite class signature to avoid internal class

Leaving the class as-is causes javac to compile two versions of each
method, one returning the public class (e.g. ServerBuilder) and one
returning the internal class (e.g., AbstractServerImplBuilder). However,
the signature is used at compile time to avoid new compilations from
referencing the internal-returning methods.

core/build.gradle

@@ -1,3 +1,9 @@
+buildscript {
+    dependencies {
+        classpath 'com.google.guava:guava:30.0-android'
+    }
+}
+
 plugins {
     id "java-library"
     id "maven-publish"
@@ -7,6 +13,10 @@ plugins {
     id "ru.vyarus.animalsniffer"
 }
 
+import static java.nio.charset.StandardCharsets.US_ASCII;
+
+import com.google.common.primitives.Bytes;
+
 description = 'gRPC: Core'
 
 evaluationDependsOn(project(':grpc-context').path)
@@ -53,7 +63,47 @@ animalsniffer {
 
 import net.ltgt.gradle.errorprone.CheckSeverity
 
+def replaceBytes(byte[] haystack, byte[] needle, byte[] replacement) {
+    int i = Bytes.indexOf(haystack, needle);
+    assert i != -1;
+    byte[] result = new byte[haystack.length - needle.length + replacement.length];
+    System.arraycopy(haystack, 0, result, 0, i);
+    System.arraycopy(replacement, 0, result, i, replacement.length);
+    System.arraycopy(haystack, i + needle.length, result, i + replacement.length, haystack.length - i - needle.length);
+    return result;
+}
+
+def bigEndian(int value) {
+    return [value >> 8, value & 0xFF] as byte[];
+}
+
+def replaceConstant(File file, String needle, String replacement) {
+    // CONSTANT_Utf8_info. https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4.7
+    byte[] needleBytes = Bytes.concat(
+        [1] as byte[], bigEndian(needle.length()), needle.getBytes(US_ASCII));
+    byte[] replacementBytes = Bytes.concat(
+        [1] as byte[], bigEndian(replacement.length()), replacement.getBytes(US_ASCII));
+    file.setBytes(replaceBytes(file.getBytes(), needleBytes, replacementBytes));
+}
+
 plugins.withId("java") {
+    compileJava {
+        doLast {
+            // Replace value of Signature Attribute.
+            // https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.9
+            project.replaceConstant(
+                destinationDirectory.file(
+                    "io/grpc/internal/AbstractManagedChannelImplBuilder.class").get().getAsFile(),
+                "<T:Lio/grpc/internal/AbstractManagedChannelImplBuilder<TT;>;>Lio/grpc/ManagedChannelBuilder<TT;>;",
+                "<T:Lio/grpc/ManagedChannelBuilder<TT;>;>Lio/grpc/ManagedChannelBuilder<TT;>;");
+            project.replaceConstant(
+                destinationDirectory.file(
+                    "io/grpc/internal/AbstractServerImplBuilder.class").get().getAsFile(),
+                "<T:Lio/grpc/internal/AbstractServerImplBuilder<TT;>;>Lio/grpc/ServerBuilder<TT;>;",
+                "<T:Lio/grpc/ServerBuilder<TT;>;>Lio/grpc/ServerBuilder<TT;>;");
+        }
+    }
+
     compileJmhJava {
         // This project targets Java 7 (no method references)
         options.errorprone.check("UnnecessaryAnonymousClass", CheckSeverity.OFF)