SF: Rename postComposition and related for clarity

The "post" in "postComposition" (and related "onPostComposition") can be
read as a preposition or a verb, making the name less easy to understand.

We believe "postComposition" is intended to mean "after composition".
More precisely, it is called after calling `present`, so rename to
"onCompositionPresented". Rename the related "postComposition" on other
classes (Layer, Scheduler) to match.

Splitting out from original motivating discussion in
Ib9d074671e32c95875ef7e0791dd95d6e595e47a.

Bug: 241285491
Bug: 259132483
Test: No new behavior; no new tests

Change-Id: I7f62728ea35aee47ebeac10dc7de82258080690e