SensorService: fix potential mutex use-before-init.

Since mRegistered is declared before mLock, it's possible to
receive a service notification callback on a binder thread
before mLock has been initialized. Prevent this by registering
in the constructor body, when we are guaranteed that the mutex
has been initialized.

Bug: 68765551
Test: many reboots on sailfish
Change-Id: I7beedc72be2a4aae961c546402f7016eacaeb3c0