Make TetheringRequest parcelable

Make TetheringRequest parcelable so we can pass it via Wifi APIs and map
Soft AP state changes to the correct request.

Bug: 216524590
Test: build
Change-Id: I63a45b14e9abc288b353159dfdcbe96fde485cb7
diff --git a/common/flags.aconfig b/common/flags.aconfig
index 010fafc..965354d 100644
--- a/common/flags.aconfig
+++ b/common/flags.aconfig
@@ -46,3 +46,9 @@
   bug: "308011229"
 }
 
+flag {
+  name: "tethering_request_with_soft_ap_config"
+  namespace: "android_core_networking"
+  description: "The flag controls the access for the parcelable TetheringRequest with getSoftApConfiguration/setSoftApConfiguration API"
+  bug: "216524590"
+}