Make DiscreteRegistry react only to offsetHistory() calls made as TestApi call.

Initialize DiscreteRegistry first as it doesn't depend on HistoricalRegistry.

There was a wrong assumption that HistoricalRegistry.offsetHistory() is used only for
testing, which was not correct, thus calls to offsetHistory() made during normal
operations of HistoricalRegistry could lead to deadlocks and corruption of timestamps
in DiscreteRegistry.

Fix: 188111112
Fix: 187490443
Bug: 187907266

Test: atest DiscreteAppopsTest

Change-Id: I97aadab497ffea5dd91e6d581bdc39e0a5df3718