Revert "libbrillo: break SecureAllocator dependence on std::vector<uint8_t>" am: 16430c43b2 am: 6d64bb179e am: ab4c5bf8a2

Original change: https://android-review.googlesource.com/c/platform/external/libbrillo/+/1353564

Change-Id: I7b164487063570927deef99d9941cb4e049fcd91