Prevent getting data from Clipboard if device is locked

Clipboard should not return data if the device is locked. This CL checks
for device locked state before returning values from get/has functions.

Test: bit -t CtsContentTestCases:android.content.cts.ClipboardManagerTest

Bug: 64934810
Change-Id: Icefac226615fe22a7735dff4ba4c3b528fb2ac12