Add feature flag for PHY plumbing

Bug: 326249470
Bug: 315241296
Test: m libbluetooth
Change-Id: I2cd5e1a679d2481e5733332a6682b4b7f46938f8

flags/gap.aconfig

@@ -92,6 +92,13 @@ flag {
     bug: "324886466"
 }
 
+flag {
+    name: "phy_to_native"
+    namespace: "bluetooth"
+    description: "Expose advertising PHY settings to native layer"
+    bug: "326249470"
+}
+
 flag {
     name: "gatt_reconnect_on_bt_on_fix"
     namespace: "bluetooth"