Change bufferhub's bufferId to int

After discussion we decided that our new system should still use int as
type of buffer id. Now the Id generator will generate int ids >= 0
instead of uint32_t ids > 0.

Remove redundant log in destructor of BufferNode, now only log when
freeId failed.

Update BufferHubIdGenerator_test.cpp to fit in current API. Add code to
cleanup generated ids in TestGenerateUniqueIncrementalID.

Test: BufferHubServer_test
Change-Id: I2018bfd009a3c311a99b9114762d0f4fbf1e3fe2
Fix: 118844348