Add null check for getActionBar()

This is done for all other fragments but missed in the root
SettingsActivity. Devices overlaying a theme that does not include an
action bar will crash when attempting to access it.

Test: atest CtsRoleTestCases
Bug: 155350832
Change-Id: I3a52baa8fcd60d880efceddf26b5d9e7ca3cd482