Add null check to DreamService.comeToFront

Dreams like doze may not have an overlay connection, which causes a
crash when invoked. This can also prevent crashes if called during
dream exit.

Bug: 359622028
Fix: 359622028
Test: atest DreamServiceTest
Flag: EXEMPT bugfix
Change-Id: I5ef49777c872a944d9247eef4eaf29acdfc035bf