Add `adb features`.

Shows the features supported by the active adb server.

Change-Id: I4a58a2ca082823c43f7a49e2c62ff80df6426756

adb/adb.cpp

@@ -956,6 +956,13 @@ int handle_host_request(const char* service, TransportType type,
         return 1;
     }
 
+    if (!strcmp(service, "features")) {
+        SendOkay(reply_fd);
+        SendProtocolString(
+            reply_fd, android::base::Join(supported_features(), '\n'));
+        return 0;
+    }
+
     // remove TCP transport
     if (!strncmp(service, "disconnect:", 11)) {
         const std::string address(service + 11);

adb/commandline.cpp

@@ -1460,6 +1460,9 @@ int adb_commandline(int argc, const char **argv) {
         fprintf(stdout, "%s", adb_version().c_str());
         return 0;
     }
+    else if (!strcmp(argv[0], "features")) {
+        return adb_query_command("host:features");
+    }
 
     usage();
     return 1;