Log when BufferClient destroyed without close

The destructor will now check mClosed and log at warning level if the
client is closed. This Could happen when the client process forget to
call close or died.

Test: run VtsHalBufferHubV1_0TargetTest, then check "adb logcat | grep
~BufferClient"
Fix: 122916649

Change-Id: I717b4ac3896a0672a5a3b81c802093f9da58c903