releasetools: Don't write outputs if not in verbose mode.

The change in [1] allows brillo_update_payload to write to stdout/stderr
directly without buffering, to ensure dumping progress to screen
continously. However, it now unconditionally writes to stdout/stderr
even if caller doesn't specify `-v`. The behavior upsets
PythonUnitTestResultParser in [2], which doesn't want any unexpected
output from stderr for result parsing.

This CL updates the logic to only give continous outputs in verbose
mode.

[1] commit 2f7e11ef
[2] https://android.googlesource.com/platform/tools/tradefederation/+/refs/heads/master/test_framework/com/android/tradefed/testtype/PythonUnitTestResultParser.java

Test: Run ota_from_target_files.py with `-v`. Check there's continous
      output from brillo_update_payload.
Test: `python -m unittest -v test_ota_from_target_files > /dev/null`
      doesn't show outputs from brillo_update_payload.
Change-Id: I3b851203ef011a5b4d982948cf52793f6fb6ad2d