atrace: fix duplicate tag 'sync'

Recent AOSP commit added a tag 'sync' for ATRACE_TAG_SYNC_MANAGER but
the tag 'sync' was already being used for "Synchronization" events.

Due to this conflict, the Synchronization events are not enabled
anymore with the name duplication. To avoid this,'sync' for
sync_manager is renamed to 'sm'

Change-Id: Ieb7b504e63f0eadc8d889af9cfed217287c75a6b