snapuserd: Rename UserSnapshotDmUserHandler to HandlerThread.

This makes it easier to type, mostly, but clarifies that it's really
just a holder object for a SnapshotHandler, and HandlerHolder would
look too odd.

Bug: 269361087
Test: snapuserd builds
Change-Id: I18aad4a58814f1e14d71cdabdd7b5ef137bd24b5
2 files changed