Fix getting selected tab

ViewPager monitors the current tab position so use that instead of
 trying to monitor the current selected tab in the ViewPager's adapter.
This appears to fix issues with the current tab not refreshing (there
 are still other refresh issues).

Patch Set 4: Expose SectionsPagerAdapter to fix build

Change-Id: I556ba06dd3d24412b151bf6715dba4581ba01b4b