libutils: String\d+ make std_string private

This method is still used by prebuilts, but making it private to start
the process of removing it from everywhere.

Test: pass
Bug: 35363681
Change-Id: I4d53f68d10c9abcca32023c4d930e99912afa697
diff --git a/healthd/BatteryMonitor.cpp b/healthd/BatteryMonitor.cpp
index 0c90a54..45b6eda 100644
--- a/healthd/BatteryMonitor.cpp
+++ b/healthd/BatteryMonitor.cpp
@@ -129,7 +129,7 @@
 }
 
 int BatteryMonitor::readFromFile(const String8& path, std::string* buf) {
-    if (android::base::ReadFileToString(String8::std_string(path), buf)) {
+    if (android::base::ReadFileToString(path.c_str(), buf)) {
         *buf = android::base::Trim(*buf);
     }
     return buf->length();