Make logger support multiple backends for the same logType.

* Previously, a Map was used to store a backend for each
  logType (using the logType as a key and backend as the
  value).
* Now each logType in the Map corresponds to an array of
  backends.

Change-Id: Ie2cf1bd30669c441e896031c491fd16b30923a98
Merge "Make logger support multiple backends for the same logType."