Refactor should_use_libusb

The function used to be called before libusb was setup.
Now is it a prompt for the state of the USB backend.

Test: NA
Bug: NA
Change-Id: Ib9be4ddb0282c9101c0362684d27b676047813eb
diff --git a/adb.cpp b/adb.cpp
index ab4e6c4..54d1854 100644
--- a/adb.cpp
+++ b/adb.cpp
@@ -1327,7 +1327,7 @@
 
     if (service == "server-status") {
         adb::proto::AdbServerStatus status;
-        if (should_use_libusb()) {
+        if (is_libusb_enabled()) {
             status.set_usb_backend(adb::proto::AdbServerStatus::LIBUSB);
         } else {
             status.set_usb_backend(adb::proto::AdbServerStatus::NATIVE);
@@ -1400,7 +1400,7 @@
     if (service == "host-features") {
         FeatureSet features = supported_features();
         // Abuse features to report libusb status.
-        if (should_use_libusb()) {
+        if (is_libusb_enabled()) {
             features.emplace_back(kFeatureLibusb);
         }
         features.emplace_back(kFeaturePushSync);