Camera: Avoid flushing uninitialized devices

Depending on timing a race is possible between two
binder threads that will eventually try to disconnect
the camera device. The first is a regular disconnect call
and the second a binderDied notification in case the
connection terminates before disconnect is able to complete.
Avoid possible instabilities and skip flushing in case the
device is no longer initialized.

Bug: 116514106
Test: Camera CTS
Change-Id: I1a958b2f80d872de89275555e83ac32576cc6792