power/stats: Index the state residency data providers

Data providers that provided data for multiple entities were being added
to the data structure as nullptr due to move semantics. Now they will
only be added once (ensuring no more nullptr entries) and an index will
map each power entity id to its corresponding data provider.

Bug: 184290936
Test: Presubmit
Change-Id: I858269beb36ba5f87bb14a228079f3abd6c2332f