Add std::string StartsWith*/EndsWith* overloads.

We should have done this from the beginning. Thanks to Windows, we're not
going to be able to switch libbase over to std::string_view any time soon.

Bug: N/A
Test: ran tests
Change-Id: Iff2f56986e39de53f3ac484415378af17dacf26b
diff --git a/init/action.cpp b/init/action.cpp
index 5fa6bec..ab51eea 100644
--- a/init/action.cpp
+++ b/init/action.cpp
@@ -358,7 +358,7 @@
     Subcontext* action_subcontext = nullptr;
     if (subcontexts_) {
         for (auto& subcontext : *subcontexts_) {
-            if (StartsWith(filename, subcontext.path_prefix().c_str())) {
+            if (StartsWith(filename, subcontext.path_prefix())) {
                 action_subcontext = &subcontext;
                 break;
             }