Add secdiscard command for secure deletion of files

This is used by LockSettingsService to delete sensitive credential files.

Bug: 34600579
Test: manual - change device lock under synthetic password, verify
      old data on disk is erased.

Change-Id: I5e11b559ad8818bd2ad2b321d67d21477aab7555
diff --git a/KeyStorage.cpp b/KeyStorage.cpp
index ddecbb8..b4f85f4 100644
--- a/KeyStorage.cpp
+++ b/KeyStorage.cpp
@@ -512,6 +512,16 @@
     return true;
 }
 
+bool runSecdiscardSingle(const std::string& file) {
+    if (ForkExecvp(
+            std::vector<std::string>{kSecdiscardPath, "--",
+                file}) != 0) {
+        LOG(ERROR) << "secdiscard failed";
+        return false;
+    }
+    return true;
+}
+
 static bool recursiveDeleteKey(const std::string& dir) {
     if (ForkExecvp(std::vector<std::string>{kRmPath, "-rf", dir}) != 0) {
         LOG(ERROR) << "recursive delete failed";