Add "repin" command to pinner service shell command.

With this CL, "adb shell cmd pinner repin" reset existing
APK pinning based on the current flag values.

For now, the only dynamic flag related to this is "pin_camera"
DeviceConfig flag.

Bug: 176838683
Test: Ran following commands and inspected the output:
   $ adb shell device_config put runtime_native_boot pin_camera false
   $ adb shell cmd pinner repin && adb shell dumpsys pinner
   $ adb shell device_config put runtime_native_boot pin_camera true
   $ adb shell cmd pinner repin && adb shell dumpsys pinner
   Also covered a couple of error cases:
   $ adb shell cmd pinner
   $ adb shell cmd pinner nosuch

Change-Id: I3229879b8637a8e0760cffb7292248e97842042f