ART: Unlink target oat file before compiling

Dex2oat must create a new file (new inode) when writing to a target
given by name, as the existing file may be in use. So unlink any
existing file first.

Bug: 22047255

(cherry picked from commit 52f0aeb8e9f16fa5c3067f6a36be701354448924)

Change-Id: Ief942c71564076e39c1e8340d5c4c55286f75896
diff --git a/dex2oat/ b/dex2oat/
index 7a23746..74d5c0c 100644
--- a/dex2oat/
+++ b/dex2oat/
@@ -1038,6 +1038,10 @@
   bool OpenFile() {
     bool create_file = !oat_unstripped_.empty();  // as opposed to using open file descriptor
     if (create_file) {
+      // We're supposed to create this file. If the file already exists, it may be in use currently.
+      // We must not change the content of that file, then. So unlink it first.
+      unlink(oat_unstripped_.c_str());
       if (oat_location_.empty()) {
         oat_location_ = oat_filename_;