core: Fix ErrorProne errors

core/src/main/java/io/grpc/internal/ManagedChannelImpl.java

@@ -259,6 +259,15 @@ public final class ManagedChannelImpl extends ManagedChannel implements WithLogI
     return balancer;
   }
 
+  // ErrorProne's GuardedByChecker can't figure out that the idleModeTimer is a nested instance of
+  // this particular instance. It is worried about something like:
+  // ManagedChannelImpl a = ...;
+  // ManagedChannelImpl b = ...;
+  // a.idleModeTimer = b.idleModeTimer;
+  // a.cancelIdleTimer(); // access of b.idleModeTimer is guarded by a.lock, not b.lock
+  //
+  // _We_ know that isn't happening, so we suppress the warning.
+  @SuppressWarnings("GuardedByChecker")
   @GuardedBy("lock")
   private void cancelIdleTimer() {
     if (idleModeTimerFuture != null) {

core/src/main/java/io/grpc/internal/ServerImpl.java

@@ -79,9 +79,10 @@ import javax.annotation.concurrent.GuardedBy;
 public final class ServerImpl extends io.grpc.Server {
   private static final ServerStreamListener NOOP_LISTENER = new NoopListener();
 
-  /** Executor for application processing. */
+  /** Executor for application processing. Safe to read after {@link #start()}. */
   private Executor executor;
-  @GuardedBy("lock") private boolean usingSharedExecutor;
+  /** Safe to read after {@link #start()}. */
+  private boolean usingSharedExecutor;
   private final InternalHandlerRegistry registry;
   private final HandlerRegistry fallbackRegistry;
   @GuardedBy("lock") private boolean started;