Fix freeId() failed in ~BufferNode

And rename UniqueIdGenerator to BufferHubIdGenerator.

Created a static getter and Use scoped static initialization for
UniqueIdGenerator.

Test: run BufferHubBuffer_test, then "adb logcat | grep bufferhub -i" to
verify that there are no error messages.
Fix: 118844348
Change-Id: Ia75ae4dac13cc709f39161c803401bcbb90c70e2