Fix leak of file descriptors in test code

skia_dm rendering through HWUI was hanging after roughly 300 tests.
logcat reports some process was unable to get any file descriptors.

When we migrated TestWindowContext into HWUI and started using PIMPL
I didn't clean up the implementation struct in the destructor. Doing
so solved the hang for me.

My guess was that BufferQueue was being leaked, and that gralloc is
backed by file descriptors, but some research suggests that Android
ought to be able to handle far more than 1k fds (ulimit -Hn returns
4096 on this device).

R=djsollen@google.com

Change-Id: I2cd9f8945cee9b22f838002e1ad687d5fe29cb97