Merge "Replace 'import mock' with unittest.mock." am: 8cf5e6de3e am: 1d2b330317 am: c440c37e0b

Original change: https://android-review.googlesource.com/c/platform/tools/treble/+/1844698

Change-Id: Ic3777ee6ca412422bc056b6a98a7a4dc61460bce