Clean up ResolverListAdapter::rebuildList().

Behaviorally, this is *almost* a pure refactoring. It does make
one very minor logic change(*) that could hypothetically fix a
race condition, although no particular bugs have ever been
observed as a result (nor do they seem especially probable), and
it's unknown what the severity would be if they ever were to occur.

Additionally, this change clarifies existing comments and adds
more inline documentation to help understand the rebuildList()
flow. Code cleanup identifies a few areas where the current design
seems a little clumsy, noted with new "TODO" comments in the code.
There's room for more improvement, but this function plays an
important role in preparing Sharesheet targets, so it's good to
bring attention to these thorny requirements.

For justifications of behavioral equivalence and other metacognitive
notes, see comments in the code review.

(*) The one behavior change is called out in code review comments;
    briefly, the line of code that sends a synchronous "results
    pending" event used to follow the line that started the async
    flow that would result in a "results complete" event. This
    could hypothetically race if the async work finished and sent
    "results complete" before we got to the "results pending" line.

Bug: 227486788
Test: `atest ChooserActivityTest` (no behavior changes expected)
Change-Id: I2e74d68579be8b34716ba2202ac2a08361008400