Set min_sdk_version for com.android.sdkext

com.android.sdkext embeds native code built against the NDK.  Set
min_sdk_version so that Soong knows to embed the NDK variant instead
of the platform variant once both exist.

Bug: 149591340
Test: m checkbuild
Change-Id: Id2f1f06981a1c303a657d35b6010e1e1d29e245f

apex/sdkextensions/Android.bp

@@ -22,6 +22,7 @@ apex {
     binaries: [ "derive_sdk" ],
     prebuilts: [ "cur_sdkinfo" ],
     manifest: "manifest.json",
+    min_sdk_version: "current",
 }
 
 apex_defaults {