Fix lint errors in GSI plugin (#160)

packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java

diff --git a/packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java b/packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java
index ec1c6e0..62fa434 100755
--- a/packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java
+++ b/packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java
@@ -117,7 +117,6 @@
 
     private static final String ERROR_REASON_EXCEPTION = "exception";
     private static final String ERROR_REASON_STATUS = "status";
-    private static final String ERROR_REASON_OPERATION_IN_PROGRESS = "operation_in_progress";
     private static final String ERROR_REASON_CONNECTION_FAILED = "connection_failed";
 
     private static final String STATE_RESOLVING_ERROR = "resolving_error";
@@ -334,7 +333,7 @@
       pendingOperation = null;
     }
 
-    private class PendingOperation {
+    private static class PendingOperation {
       final String method;
       final Result result;