Use std::optional instead of std::unique_ptr

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

Bug: 149784838
Test: m
Change-Id: Ide63196c6350df30bf43dca15c9334e2f4b36ed7
diff --git a/include/media/MicrophoneInfo.h b/include/media/MicrophoneInfo.h
index 2287aca..0a24b02 100644
--- a/include/media/MicrophoneInfo.h
+++ b/include/media/MicrophoneInfo.h
@@ -205,11 +205,11 @@
 private:
     status_t readFloatVector(
             const Parcel* parcel, Vector<float> *vectorPtr, size_t defaultLength) {
-        std::unique_ptr<std::vector<float>> v;
+        std::optional<std::vector<float>> v;
         status_t result = parcel->readFloatVector(&v);
         if (result != OK) return result;
         vectorPtr->clear();
-        if (v.get() != nullptr) {
+        if (v) {
             for (const auto& iter : *v) {
                 vectorPtr->push_back(iter);
             }