Parcel: add "optional" variant of writeParcelableVector

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

Adding "optional" variant of writeParcelableVector which was missed
when adding optional variants of read/write methods before deprecating
"unique_ptr" variants.

Bug: 149784838
Test: m
Change-Id: I2f6781ea5e367d414a371c55a3eb8fe794c144d0