releasetools: merge_target_files sets verbose in main.

The flag would otherwise be turned on unconditionally even when
importing merge_target_files as a module. As a result, `atest
releasetools_test` was broken / flaky prior to this change, as
PythonUnitTestResultParser doesn't like unexpected outputs in stderr.

Test: `atest releasetools_test releasetools_py3_test`
Change-Id: Ifd614c797d2cee3b33e4c2a7ad8e598850e9d60c