Replace security_context_t type

security_context_t has been marked as deprecated in libselinux from
version 3.2. Update to the `char*` type.

Bug: 190808996
Test: m
Change-Id: I6f40e161251c79893d41e12c368715736578aacc
diff --git a/Utils.cpp b/Utils.cpp
index 4635975..f9f3058 100644
--- a/Utils.cpp
+++ b/Utils.cpp
@@ -68,10 +68,10 @@
 namespace android {
 namespace vold {
 
-security_context_t sBlkidContext = nullptr;
-security_context_t sBlkidUntrustedContext = nullptr;
-security_context_t sFsckContext = nullptr;
-security_context_t sFsckUntrustedContext = nullptr;
+char* sBlkidContext = nullptr;
+char* sBlkidUntrustedContext = nullptr;
+char* sFsckContext = nullptr;
+char* sFsckUntrustedContext = nullptr;
 
 bool sSleepOnUnmount = true;
 
@@ -702,7 +702,7 @@
 }
 
 status_t ForkExecvp(const std::vector<std::string>& args, std::vector<std::string>* output,
-                    security_context_t context) {
+                    char* context) {
     auto argv = ConvertToArgv(args);
 
     android::base::unique_fd pipe_read, pipe_write;