Rename "leader" display to "pacesetter"

"Leader" became ambiguous once we began to consider that there are two
orthogonal concepts that "leader" seems to imply:
- The display with the highest refresh rate (originally called "leader")
- The display that is refreshed first in a given update (assuming it's
  being updated)

To better distinguish between the two, we've come up with new names:
- pacesetter
- front runner (for the second concept)

Change uses of "leader" to "pacesetter" throughout SF and Scheduler.

Bug: 255635821
Bug: 256196556
Test: m
Change-Id: Ia02c35dd442d310695f1ce873fbaf30910c12419