Use FgThread for callbacks in RemoteRoleControllerService.

Previously RemoteRoleControllerService was calling callbacks on
BackgroundThread, which was causing disk writes delaying queued binder
calls significantly. FgThread is the right thread for this and should
solve this issue.

Bug: 110557011
Test: manual
Change-Id: I6b7a55c2197b1f86572705e6df46cd3f18ff91f7