BQ: Make QueueBufferOutput implement Flattenable

It will need to support file descriptors soon.

Also:
* Make it's members public and clean up
    inflate/deflate usage.
* Send Fence::NO_FENCE over Binder from dequeue
    to indicate lack of fence.

Test: adb shell /data/nativetest/libgui_test/libgui_test
--gtest_filter=*GetFrameTimestamps*

Change-Id: Ia9e4600424a89ce026ba27e8aaed27a6bab860a4