Add buffer result for generating vendor configs

Add buffers to verify the success of generating and restoring vendor
configs.

Bug: 281835821
Test: atest android.car.apitest.CarPropertyManagerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6791803d4ad568c8ba66d20199e84746f412df23)
Merged-In: Ic3cb8bf3e606847ff55d719900ad0b764b56aaf4
Change-Id: Ic3cb8bf3e606847ff55d719900ad0b764b56aaf4