Prevent callback registration when back flag is disabled

When the enableOnBackInvokedCallback is set to false (or not set),
registering an OnBackInvokedCallback should be a no-op to avoid
overriding the default compat callback.

Test: Manual testing registering a callback on an app with the flag
disabled and doing a back gesture. Currently we don't have test
executing a back gesture so automated tests are not possible

Bug: 235206960

Change-Id: I54d843f11130a78ed5a68cbe4722e601a2086ee1
Merged-In: I54d843f11130a78ed5a68cbe4722e601a2086ee1
(cherry picked from commit aa48dc3c)