Clean up external HAL reference implementation.

This CL cleans up the reference external HAL implementation.
Specifically, it makes the following changes:

  - Removes mSupportBufMgr from ExternalCameraDeviceSession as it never
    needed to be used.
  - Makes ExternalCameraOfflineSession final to prevent other classes
    from accidentally inheriting from it. ExternalCameraOfflineSession
    calls the virtual function `close()` on destruction which depends on
    the implementation details of other virtual functions.

Bug: 219974678
Test: Builds successfully
Change-Id: I645f8164cd2426219943d9407f0c282c468c90d3