[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/transport_test.cpp b/transport_test.cpp
index 00beb3a..a9ada4a 100644
--- a/transport_test.cpp
+++ b/transport_test.cpp
@@ -66,7 +66,7 @@
     ASSERT_TRUE(t.has_feature("bar"));
 
     t.SetFeatures(FeatureSetToString(FeatureSet{"foo", "bar", "foo"}));
-    ASSERT_EQ(2U, t.features().size());
+    ASSERT_LE(2U, t.features().size());
     ASSERT_TRUE(t.has_feature("foo"));
     ASSERT_TRUE(t.has_feature("bar"));