zygote: initialize selinux app contexts before fork()

Initializing before zygote fork() results in only doing this once,
which appears to be the original intent of seapp_context_init().
However, since it was indirectly called after fork() it was
happening on during every app launch.

Bug: 129704390
Test: ps -AZ; verify that apps have correct context
Test: with "#define DEBUG 1" in libselinux to verify that
selinux_android_seapp_context_reload() is only called once
in zygote.

Change-Id: Ibd6f0dda065f35def99f39ad9d44829a6b60cdc9

core/jni/com_android_internal_os_Zygote.cpp

@@ -1634,6 +1634,8 @@ static void com_android_internal_os_Zygote_nativeSecurityInit(JNIEnv*, jclass) {
   // security_getenforce is not allowed on app process. Initialize and cache
   // the value before zygote forks.
   g_is_security_enforced = security_getenforce();
+
+  selinux_android_seapp_context_init();
 }
 
 static void com_android_internal_os_Zygote_nativePreApplicationInit(JNIEnv*, jclass) {