adb: Avoid extra std::string copies

When a function argument takes a std::string, it's inefficient to
pass std::string::c_str(), since that creates an additional copy.
So we change these calling sites.

Test: TreeHugger
Change-Id: I1c0fde7275eb3ebd3baf05ec62581b0243655608
diff --git a/bugreport_test.cpp b/bugreport_test.cpp
index 72ca59a..a6be203 100644
--- a/bugreport_test.cpp
+++ b/bugreport_test.cpp
@@ -136,7 +136,7 @@
 
     void ExpectBugreportzVersion(const std::string& version) {
         EXPECT_CALL(br_, SendShellCommand("bugreportz -v", false, _))
-            .WillOnce(DoAll(WithArg<2>(WriteOnStderr(version.c_str())),
+            .WillOnce(DoAll(WithArg<2>(WriteOnStderr(version)),
                             WithArg<2>(ReturnCallbackDone(0))));
     }