Rename isValidSubtypeId() to isValidSubtypeHashCode()

This is a mechanical rename of

  SubtypeUtils#isValidSubtypeId()

to

  SubtypeUtils#isValidSubtypeHashCode()

as it takes a subtype hashcode rather than a subtype index.

There must be no behavior change.

Bug: 346662143
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I7c5d846ff88a621ee495c0a2e029464891bdd47c