Set min_sdk_version for the test payload

We now have GTS tests that run on previous Android releases. Because we
didn't set the min_sdk_version of the native library, loading the
library in Microdroid failed with:
'cannot locate symbol "AVmPayload_readRollbackProtectedSecret"'.

Setting min_sdk_version makes APIs that were added after the
min_sdk_version use weak linking, so loading the library will succeed.

This patch alone is not enough to fix the GTS test failures, since
loading still fails with a different error which I'm still looking into:
'dlopen failed: cannot locate symbol "AIBinder_Class_setTransactionCodeToFunctionNameMap'

Bug: 389781453
Bug: 388702766
Test: atest MicrodroidTestApp.GTS
Change-Id: If14d82b92484bfe9d206b8d86d5ea07fad9ecfe6
diff --git a/tests/testapk/Android.bp b/tests/testapk/Android.bp
index d0e045b..99300e2 100644
--- a/tests/testapk/Android.bp
+++ b/tests/testapk/Android.bp
@@ -173,6 +173,8 @@
         "liblog",
         "libprotobuf-cpp-lite-ndk",
     ],
+    // We've added support for updatable payloads in Android U.
+    min_sdk_version: "UpsideDownCake",
 }
 
 cc_library_shared {