Add optional emitFirstEvent param to setChanges()

There are situations where you really don't want to treat the first
event as a change; for these cases, you can use

    setChanges(emitFirstEvent = false)

Unfortunately, due to the usage of distinctUntilChanged() in the
definition of setChanges(), there is no (obvious) way to recover this
behavior on the client-side by dropping / filtering events, so we need
to add this option to the method signature.

Bug: 241121499
Test: atest SetChangesFlowTest
Change-Id: I65ce50c12fb6299fa490887555e40db69f0eebdf