Add AccentActivity to make it easier to get started

Created an abstract Activity which handles themeing from XML without
needing to add any methods or fields to the implementation.
Overriding the accent color programmatically requires overriding a
single method in the Activity.

Resulting changes:
- Added AccentActivity
- Added constructor to AccentHelper
- Refactor HoloAccentExample to not rely on android-support-v4 for
  Fragments and FragmentActicity.
- Bumped build tool versions
- Changed support library dependency