SystemApi is @Repeatable

This allows us to annotate a class as below:

/** @hide */
@SystemApi
@SystemApi(client=MODULE_APPS)
public class SomeClass {
    /** @hide */ @SystemApi
    public void foo() {...}

    /** @hide */ @SystemApi(client=MODULE_APPS)
    public void bar() {...}
}

SomeClass is already annotated as @SystemApi. And we want to make bar()
method in it as SystemApi(MODULE_APPS). To do so, the containing class
SomeClass has additionally to be annotated as SystemApi(MODULE_APPS),
resulting SystemApi annotation to be repeated.

Bug: N/A
Test: add @SystemApi(client=MODULE_APPS) to an hidden method of an
existing SystemApi class (e.g. SystemProperties.set()) and execute `m
updata-api`.
-> api/module-app-current.txt is updated while
api/system-current.txt is unchanged

Change-Id: Ifd4d32a6983cfc38f0dd13618652439f6162e0d3

core/java/android/annotation/SystemApi.java

@@ -23,6 +23,7 @@ import static java.lang.annotation.ElementType.METHOD;
 import static java.lang.annotation.ElementType.PACKAGE;
 import static java.lang.annotation.ElementType.TYPE;
 
+import java.lang.annotation.Repeatable;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
@@ -40,6 +41,7 @@ import java.lang.annotation.Target;
  */
 @Target({TYPE, FIELD, METHOD, CONSTRUCTOR, ANNOTATION_TYPE, PACKAGE})
 @Retention(RetentionPolicy.RUNTIME)
+@Repeatable(SystemApi.Container.class) // TODO(b/146727827): make this non-repeatable
 public @interface SystemApi {
     enum Client {
         /**
@@ -86,4 +88,14 @@ public @interface SystemApi {
      * The process(es) that this SystemAPI is available
      */
     Process process() default android.annotation.SystemApi.Process.ALL;
+
+
+    /**
+     * Container for {@link SystemApi} that allows it to be applied repeatedly to types.
+     */
+    @Retention(RetentionPolicy.RUNTIME)
+    @Target(TYPE)
+    @interface Container {
+        SystemApi[] value();
+    }
 }