Rewrite BufferHubBuffer_test to avoid test crash

We noticed that ASSERT_XX is not going to terminate the tests if called
inside a hidl callback. Therefore, this CL is moving the checks outside
of the callback.

Test: BufferHubBuffer_test (passed)
Bug: none
Change-Id: I0960c801e6a4dd007bb1c1138871cd4b9547f68f