binder_ndk: use std::optional

Since Android R, Parcel provides read/write methods for "@nullable"
values using std::optional.

Bug: 149784838
Test: m
Test: CtsNdkBinderTestCases
Change-Id: If8f3e4c658766ec197471c0994b0611cdb076944
diff --git a/libs/binder/ndk/parcel.cpp b/libs/binder/ndk/parcel.cpp
index 3e3eda1..ec7c7d8 100644
--- a/libs/binder/ndk/parcel.cpp
+++ b/libs/binder/ndk/parcel.cpp
@@ -261,7 +261,7 @@
 }
 
 binder_status_t AParcel_readParcelFileDescriptor(const AParcel* parcel, int* fd) {
-    std::unique_ptr<ParcelFileDescriptor> parcelFd;
+    std::optional<ParcelFileDescriptor> parcelFd;
 
     status_t status = parcel->get()->readParcelable(&parcelFd);
     if (status != STATUS_OK) return PruneStatusT(status);