Fix OverlayConfigTest minSdkVersion

With a fix to ApkLite parsing, targetSdk gets coerced to a minimum
of the minSdkVersion. And by default the build system sets the min to
the dev version, so the target would be brought up to 10000.

The test expects 21, so make the min just below that at 20.

Bug: 189988918

Test: atest com.android.internal.content.OverlayConfigTest

Change-Id: Iec7af27012c207e8cc888b6d16fc71e5e47bc73a