Fix PinnerService SecurityException at boot

We were passing "system" as the callingPackage argument when calling
registerUidObserver, which is apparently invalid. null is a valid option
here and seems to pin everything as expected, so this change updates
this call to avoid the SecurityException.

Note that this SecurityException does not seem to have affected the
actual pinning of the expected files - we still pin the same files
before and after this change.

Bug: 147355976
Test: No longer see the SecurityException at boot. Running dumpsys
pinner at boot shows that the same files are pinned as before this
change, including the Camera app but not including a Launcher app.
After using the four-corner tap to bypass the setup wizard, the Launcher
app is pinned. This indicates that the uid observer worked correctly to
re-pin the Launcher app after it replaces the setup wizard.

Change-Id: I2cd845cb5fad3de251d110141dcd1af9ec3aa234