BatteryStatsService: Allow unknown wakeup string

Also clean up wakeup reason concatenation code. Cannot find a reason for
there to be restrictions on the wakeup reason strings that this service
is enforcing. It is the job of the Suspend Control Service to ensure
valid wakeup reasons are supplied.

Bug: 174106197
Test: Generated a bugreport and viewed wakeups in historian
Change-Id: I4cc79bec0e1face40d51e35dec1d443fa1165959