Camera: Avoid possible race condition during provider init

The current sequence of registering for provider notifications
and trying to add a legacy provider could introduce a race
condition leading to a deadlock. The "onRegistration" callback
will try to acquire "mInterfaceMutex" which is still held during
 the execution of "addProvider" for legacy. However "addProvider"
could get suspended waiting on the HIDL provider interface. If this
happens and the same thread that unblocks the service query tries
to notify the provider manager, then we will end up in a deadlock.
To avoid this, the initialization sequence can be modified a bit.
First add the legacy provider and then register for notifications.

Bug: 35096594
Test: Manual using stop/start of cameraserver
Change-Id: Ia381ae6bf567525cabd3f51246a192ddac37b7f8