Move 712-varhandle-invocations to API level 28

Bug: 65168732
Test: art/test/run-test  --no-relocate --64 --build-with-javac-dx --runtime-option -Xcheck:jni --never-clean --host 712-varhandle-invocations
Change-Id: Ie8cab287541dd82324cc8bb8365bd31c552d0518
diff --git a/test/etc/default-build b/test/etc/default-build
index dd55602..8bb898c 100755
--- a/test/etc/default-build
+++ b/test/etc/default-build
@@ -145,7 +145,7 @@
 declare -A DX_EXPERIMENTAL_ARGS
 DX_EXPERIMENTAL_ARGS["method-handles"]="--min-sdk-version=26"
 DX_EXPERIMENTAL_ARGS["parameter-annotations"]="--min-sdk-version=25"
-DX_EXPERIMENTAL_ARGS["var-handles"]="--min-sdk-version=26"
+DX_EXPERIMENTAL_ARGS["var-handles"]="--min-sdk-version=28"
 
 while true; do
   if [ "x$1" = "x--dx-option" ]; then