Move BufferHubBase::Lock and ::Unlock from public function to private helper.

User should be using GraphicBuffer/AHardwareBuffer::Lock/Unlock to lock/unlock
for CPU access to the data in the buffer.

BufferHubBase::Lock/Unlock exist were because this legacy class has IonBuffer,
which wraps a GraphicBuffer. The dependency will be reverted in the future, where
BufferHubBuffer backs GraphicBuffer.

Bug: None
Test: m, mma, vrflinger_test
Change-Id: I3e3287529326dce3ae57e344c5c738b898c3d44d