Ueventd: Fix a corner case in ReadUevent() that triggers duplicate firmware loading.

Presently, within ReadUevent(), true is returned for a successful case
as well as for the case where we read an invalid uevent (overflowed
buffer)where the Uevent object is not cleared, and the caller calls the
callback (with the earlier stored uevent object),leading to duplicate
firmware loading.

Uevent uevent;
while (ReadUevent(&uevent)) {
  if (callback(uevent) == ListenerAction::kStop) return;
}

Scenario:
1. Proper Uevent received and callback is called (firmware loading
   is triggered).
2. Overflowed uevent is received as part of the same ReadUevent session,
   ReadUevent() returns true, but the uevent object is not cleared and
   still has earlier event values.
3. Callback is called again, leading to duplicate firmware load.

Handle it by adding explicit return codes to let the caller know if the
uevent read is invalid, and the caller can ignore it and read
further pending uevents.

Bug: 161580785

Change-Id: I09e80052337fd1495b968dc02ecff5ceb683da18