Address const issue in preparation for libcxx rebase.

Change-Id: I80c3349ee93d2b3f16c6fadc7f34b2bf4f2cc0d3
diff --git a/adb/shell_service.cpp b/adb/shell_service.cpp
index ce10708..3eeed34 100644
--- a/adb/shell_service.cpp
+++ b/adb/shell_service.cpp
@@ -246,7 +246,7 @@
         char** current = environ;
         while (char* env_cstr = *current++) {
             std::string env_string = env_cstr;
-            char* delimiter = strchr(env_string.c_str(), '=');
+            char* delimiter = strchr(&env_string[0], '=');
 
             // Drop any values that don't contain '='.
             if (delimiter) {