More tetheroffload VTS fixes for over-specified tests.

[1] Call addDownstream before removeDownstream in affirmative tests

In order to properly test removeDownstream() where the HAL might
reasonably reject downstreams not previously added we now call
addDownstream() first (for affirmative tests).

[2] Clarify when stopOffload() return values can be safely ignored.

Test: as follows
    - make vts -j30 BUILD_GOOGLE_VTS=true && \
      vts-tradefed run commandAndExit vts \
          --skip-all-system-status-check \
          --primary-abi-only \
          --skip-preconditions \
          --module VtsHalTetherOffloadControlV1_0Target \
          -l DEBUG
      still doesn't pass but it's better than before  :)
Bug: 65270149
Change-Id: I27a574bd2110e3a1626de343f6b57b9efb8cdf83