Flag declaration file for Nearby

Using the same container value as other flags in Connectivity

Bug: 324993608
Test: TH
Change-Id: I6c8bbb4401ced54207ed352ce08c5930de4d6ab1

nearby/flags/nearby_flags.aconfig

+package: "com.android.nearby.flags"
+container: "system"
+
+flag {
+    name: "powered_off_finding"
+    namespace: "nearby"
+    description: "Controls whether the Powered Off Finding feature is enabled"
+    bug: "307898240"
+}