Tweaks to tetheroffload VTS: fixes and fd leaks

Specifically:
    - always use hidl_handle::setTo(..., true) to force fd ownership
      (and thereby cleanup in hidl_handle destructor)
    - initialize config HAL multiple times to try to trigger fd leaking bugs

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 VtsHalTetherOffloadConfigV1_0Target -l INFO

      I/ResultReporter: Invocation finished in 48s. PASSED: 4, FAILED: 0, MODULES: 1 of 1

    - 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 INFO
Bug: 29337859
Bug: 32163131
Bug: 64976634
Bug: 65270149
Bug: 65529504
Change-Id: Ib47b35a01092bb2052d241f7bba951bbf7691b66