Camera server: Move shutter notify callback outside in-flight mutex

This further reduces contention on the in-flight queue mutex with
calls like getExpectedInFlightDuration().
On a test trace recording 1080p60 video on Pixel, the average duration
of getExpectedInFlightDuration reduced from 12 to 5 us, with a much
narrower histogram as well.

Test: Manual trace inspection from 1080p60 recording, atest CtsCameraTestCases
Bug: 315526878
Change-Id: I50f2c835247b22a6c9acb637740814b0d721ceea