Fix NPE in ClipboarService.

Check if primaryClip is null or not before trying to set the timestamp.

Fixes: 35144954
Test: cts-tradefed run singleCommand cts-dev --module -t \
      com.android.cts.devicepolicy.ManagedProfileTest#testCrossProfileCopyPaste

Change-Id: If322626553f42dda56a42adc2c6e4d206715a673