alts: add close to TsiHandshaker to avoid resource leak for some impls

alts/src/main/java/io/grpc/alts/internal/AltsTsiHandshaker.java

@@ -192,4 +192,9 @@ public TsiFrameProtector createFrameProtector(int maxFrameSize, ByteBufAllocator
   public TsiFrameProtector createFrameProtector(ByteBufAllocator alloc) {
     return createFrameProtector(AltsTsiFrameProtector.getMaxAllowedFrameBytes(), alloc);
   }
+
+  @Override
+  public void close() {
+    handshaker.close();
+  }
 }

alts/src/main/java/io/grpc/alts/internal/NettyTsiHandshaker.java

@@ -149,4 +149,8 @@ TsiFrameProtector createFrameProtector(ByteBufAllocator alloc) {
     unwrapper = null;
     return internalHandshaker.createFrameProtector(alloc);
   }
+
+  void close() {
+    internalHandshaker.close();
+  }
 }

alts/src/main/java/io/grpc/alts/internal/TsiHandshakeHandler.java

@@ -185,4 +185,9 @@ private void sendHandshake(ChannelHandlerContext ctx) throws GeneralSecurityExce
       }
     }
   }
+
+  @Override
+  protected void handlerRemoved0(ChannelHandlerContext ctx) throws Exception {
+    handshaker.close();
+  }
 }

alts/src/main/java/io/grpc/alts/internal/TsiHandshaker.java

@@ -106,4 +106,9 @@ public interface TsiHandshaker {
    * @return a new TsiFrameProtector.
    */
   TsiFrameProtector createFrameProtector(ByteBufAllocator alloc);
+
+  /**
+   * Closes resources.
+   */
+  void close();
 }

alts/src/test/java/io/grpc/alts/internal/AltsProtocolNegotiatorTest.java

@@ -479,6 +479,11 @@ public TsiFrameProtector createFrameProtector(int maxFrameSize, ByteBufAllocator
       protectors.add(protector);
       return protector;
     }
+
+    @Override
+    public void close() {
+      delegate.close();
+    }
   }
 
   private static class InterceptingProtector implements TsiFrameProtector {

alts/src/test/java/io/grpc/alts/internal/FakeTsiHandshaker.java

@@ -226,4 +226,9 @@ public TsiFrameProtector createFrameProtector(int maxFrameSize, ByteBufAllocator
   public TsiFrameProtector createFrameProtector(ByteBufAllocator alloc) {
     return createFrameProtector(AltsTsiFrameProtector.getMaxAllowedFrameBytes(), alloc);
   }
+
+  @Override
+  public void close() {
+    // No-op
+  }
 }