Collapses ForegroundServiceControllerImpl into interface.

ForegroundServiceControllerImpl was the only implementation of
ForegroundServiceController; this change eliminates the interface so
that the class can be broken up into more granular components in a
future CL to help break circular dependencies.  No functional changes.

Test: atest SystemUITests, manual
Change-Id: Iabe8ae1dbc42769d5b3e93ebe2249cdc5c3e4e77