Do not require package visibility when obtaining SessionTokens

The only reason this is required at the moment is to set the
process UID field in the token, that is supposed to make it easier
for controller apps to identify the session. However, if this
visibility is not provided, it shouldn't stop us from creating
the controller for this session.

Also docuement more clearly what UID means in this context.

PiperOrigin-RevId: 490184508

libraries/session/src/main/java/androidx/media3/session/SessionToken.java

@@ -36,6 +36,7 @@
 import androidx.annotation.Nullable;
 import androidx.media.MediaBrowserServiceCompat;
 import androidx.media3.common.Bundleable;
+import androidx.media3.common.C;
 import androidx.media3.common.MediaLibraryInfo;
 import androidx.media3.common.util.UnstableApi;
 import com.google.common.collect.ImmutableSet;
@@ -179,7 +180,11 @@ public String toString() {
     return impl.toString();
   }
 
-  /** Returns the uid of the session */
+  /**
+   * Returns the UID of the session process, or {@link C#INDEX_UNSET} if the UID can't be determined
+   * due to missing <a href="https://developer.android.com/training/package-visibility">package
+   * visibility</a>.
+   */
   public int getUid() {
     return impl.getUid();
   }
@@ -393,8 +398,7 @@ private static int getUid(PackageManager manager, String packageName) {
     try {
       return manager.getApplicationInfo(packageName, 0).uid;
     } catch (PackageManager.NameNotFoundException e) {
-      throw new IllegalArgumentException(
-          "Cannot find package " + packageName + " or package is not visible", e);
+      return C.INDEX_UNSET;
     }
   }