Fix WorkSource clientId detection

Initialize mActiveWakeLockWorkSource so there can be no chance
of two identical worksources being added/removed with each other.

In existing WorkSource logic, if worksources do equal each other,
then size decrements while uid and name exist - this causes
the clientId to be null since it is empty, even though the
clientId value exists. Test added in WorkSourceTest at ag/4627499
and aosp/729235.

Bug: 111652977
Test: locally, ag/4627499, aosp/729235
Change-Id: If23a20f26d916ef059ff02b727f8560de5f68e00