[adb] Don't copy features set on each get()

The features are already cached in a static and don't change.
Let's return the cached set instead of copying it every time.

Bug: 150183149
Test: manual
Change-Id: Ifdca852cc3b32e09e50ea4771f7878987c46cce8
diff --git a/adb/adb.cpp b/adb/adb.cpp
index 0518e9d..08d3904 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -1198,9 +1198,9 @@
         FeatureSet features = supported_features();
         // Abuse features to report libusb status.
         if (should_use_libusb()) {
-            features.insert(kFeatureLibusb);
+            features.emplace_back(kFeatureLibusb);
         }
-        features.insert(kFeaturePushSync);
+        features.emplace_back(kFeaturePushSync);
         SendOkay(reply_fd, FeatureSetToString(features));
         return HostRequestResult::Handled;
     }