Remove obsolete logic in SettingsActivity.

mIsShowingDashboard is always false, it used to be true when we are
displaying homepage, but now homepage is hosted in a entirely different
activity.

so all related logic can now be removed.

Test: robotests

Misc clean up: remove unused colors

Test: rebuild, color-lint
Change-Id: I1e1628c1e9606c2b7dc40ef3c21d4ed1391a8c03