Revert "libbrillo: break SecureAllocator dependence on std::vector<uint8_t>" am: 16430c43b2 am: 6d64bb179e am: ab4c5bf8a2 am: 6565bfd206

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

Change-Id: Ia4e1988ac461171b69bae5547450d7fbf3f47722