SF: Tidy naming of mode set FSM

Rename {Desired,Upcoming}ActiveMode to {Desired,Pending}Mode for clarity
that the FSM is in one of desired/pending/active states.

Rename clearDesiredActiveModeState to dropModeRequest, as its purpose is
to undo the effect of a DisplayModeRequest, regardless of FSM state.

Rename desiredActiveModeChangeDone to applyActiveMode, as it pertains to
applying the DisplayModeRequest that has become active to the Scheduler.
This function will later be moved from SF to Scheduler.

Bug: 255635711
Test: presubmit
Change-Id: Ie7fd9a353e38325d7b8dfd1d1143c7d52e5ad58e