More theming fixes.

Updated the importance seekbar to use colorAccent instead of a hardcoded
color.

Refactored the various gear icons to all use ic_settings (a rename of
ic_bt_config). Deleted the png version (ic_sysbar_quicksettings).

Cleaned up the add icon. Now there are two vector drawables (one white,
one tinted with the accent color). Deleted the existing pngs and updated
the usages to use one of the two vector drawables. Deleted the theme
attr for ic_menu_add that had no usages.

BUG: 28760785

Change-Id: Icd5561d5b3f5ddee8aab78a3da7daa2dca0e30e3