set sdk_version to backstage_power_flags_lib

This change correct sdk_version for backstage_power_flags_lib. Before
the sdk_version is not set, and by default the library will be built
against the implementation of the framework. Set the sdk_version to the
right scope.

Test: presubmit
Bug: 327965477
Change-Id: Ifd2e62e42f3b42ffd19e18fc239e576b7c171dbd
diff --git a/services/core/java/com/android/server/power/Android.bp b/services/core/java/com/android/server/power/Android.bp
index 607d435..863ff76 100644
--- a/services/core/java/com/android/server/power/Android.bp
+++ b/services/core/java/com/android/server/power/Android.bp
@@ -9,4 +9,5 @@
 java_aconfig_library {
     name: "backstage_power_flags_lib",
     aconfig_declarations: "backstage_power_flags",
+    sdk_version: "system_current",
 }