subcontext: Change a std::string argument into std::string_view

This change prevents that a later CL will trigger the following warning:

parameter 'test_context' is passed by value and only copied once; consider
moving it to avoid unnecessary copies

Change-Id: If5837ee6438efdf194781de041779c1089897789
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/init/subcontext.h b/init/subcontext.h
index 8acc032..93ebace 100644
--- a/init/subcontext.h
+++ b/init/subcontext.h
@@ -36,8 +36,10 @@
 
 class Subcontext {
   public:
-    Subcontext(std::vector<std::string> path_prefixes, std::string context, bool host = false)
-        : path_prefixes_(std::move(path_prefixes)), context_(std::move(context)), pid_(0) {
+    Subcontext(std::vector<std::string> path_prefixes, std::string_view context, bool host = false)
+        : path_prefixes_(std::move(path_prefixes)),
+          context_(context.begin(), context.end()),
+          pid_(0) {
         if (!host) {
             Fork();
         }