LArRawChannelMonAlg: Change name of event-info ReadHandle to avoid name-clash with base-class

Fix a bug related to a typo & a name-clash: The LArRawChannelMongAlg.h declared a ReadDecorHandleKey<EventInfo> called m_eventInfoKey. But the method LArRawChannelBuilderAlg::initialize() called m_EventInfoKey.initialize() (double-)initializing a ReadHandleKey declared in the AthenaMonitorAlg base-class and leaving the m_eventInfoKey (non-capital E) uninitialized. Solution: Call the variable m_larFlagKey that better describes it's purpose (handle to the EventInfo.larFlag decoration) and is very different from any variable declared in the base-class.