ActionBar is deprecated in favor of Toolbar, which was added in API 21. However, the base theme for Android Studio's templates still seems to use ActionBar-based themes.
The solution is not to downgrade your build tools version, but rather to change your app's base theme. If you downgrade your build tools, you will miss out on support for the latest features added in Android 5.1 and later.
Your base theme is listed in AndroidManifest.xml under application/android:theme.
Go to res/values/styles.xml and change the parent attribute to a different base theme that doesn't rely on the deprecated ActionBar. In the unlikely case your app is API 21+, you can use Theme.Material. Otherwise, you'll need to use Theme.AppCompat.NoActionBar or define your own base theme.
Here's a styles.xml example:
<style name="AppTheme.Base" parent="Theme.AppCompat">
<item name="colorPrimary">@color/colorPrimary</item>
<item name="colorPrimaryDark">@color/colorPrimaryDark</item>
<item name="android:windowNoTitle">true</item>
<item name="windowActionBar">false</item>
</style>
If you use this, be sure to put a Toolbar element in your XML layouts where you want the ActionBar to appear. You'll need to customize the Toolbar with child XML tags. I recommend using the <include>
tag. As it says in the Toolbar documentation, Google's new design principles call for a change in visual style:
In modern Android UIs developers should lean more on a visually distinct color scheme for toolbars than on their application icon. The use of application icon plus title as a standard layout is discouraged on API 21 devices and newer.
android.support.v7.widget.Toolbar is in the support library for pre-API 21 devices.
EDIT: As the first comment points out, Google has a blog explaining how to use the new Toolbar. http://android-developers.blogspot.kr/2014/10/appcompat-v21-material-design-for-pre.html