Fix Gravity toString wrong message issue

1. Bit calculation was not correct before as there are multiple bits
that can be set.
2. Changed a duplicate CLIP message as well.
3. And add an empty check.

Test: In 'dumpsys window' the gravity of the window should be correct.
Change-Id: I4b699e5d6d5d7c2beb8ac12cbd78cf2f81e70b3f