VideoDecTest: stop managing memory manually

We can get the same effect as `malloc`'ing here by using `std::vector`.
Further, with `std::vector`, we don't have theoretical issues with
`ASSERT_EQ(_, nullptr)` failing, which would lead to us potentially
leaking one of the allocations here.

Bug: 206470603
Test: TreeHugger
Change-Id: I7568317c74f6403da3d0755c97b3fd418b9d7cdb