Replace ImageDecoder.sApiLevel with a SystemApi helper

Bug: 152322291
Test: CtsGraphicsTestCases

ImageDecoder is moving to a mainline module, so the system can no longer
access the @hidden sApiLevel without making it @SystemApi. Instead of
promoting the existing API, provide a common class to be used by any UI
rendering class that needs to access the targetSdkVersion. Switch AVD to
use the new class, which is a SystemApi.

Change-Id: Ie7172fb93364a1e04ab844b8fa64887bf9d8b005