ART: Add FdFile constructors

Make Open protected, and expose constructors instead. Add a move
constructor and move assignment operator.

Add OS functions that return the FdFile non-pointer version.

Add tests.

Bug: 21192156
Test: m test-art-host
Test: m test-art-target (shamu)
Change-Id: I83e390edde7cd37c900e9d5c3e4d21da22981b3f
diff --git a/runtime/utils.cc b/runtime/utils.cc
index 6a50b8e..3f779df 100644
--- a/runtime/utils.cc
+++ b/runtime/utils.cc
@@ -136,8 +136,8 @@
 }
 
 bool ReadFileToString(const std::string& file_name, std::string* result) {
-  File file;
-  if (!file.Open(file_name, O_RDONLY)) {
+  File file(file_name, O_RDONLY, false);
+  if (!file.IsOpened()) {
     return false;
   }
 
@@ -155,8 +155,8 @@
 }
 
 bool PrintFileToLog(const std::string& file_name, LogSeverity level) {
-  File file;
-  if (!file.Open(file_name, O_RDONLY)) {
+  File file(file_name, O_RDONLY, false);
+  if (!file.IsOpened()) {
     return false;
   }