releasetools: Move build_image.RunCommand into common.py.

build_image.RunCommand and common.Run are essentially doing the same
work, but with different interfaces and return values. This CL
consolidates them by moving build_image.RunCommand into common.py as
common.RunAndCheckOutput, with the former calling common.Run internally.

common.RunAndCheckOutput checks the exit code: it returns the output
string on success, or raises common.ExternalError otherwise.

Test: `m dist` with aosp_marlin-userdebug
Test: python -m unittest test_build_image
Test: python -m unittest test_common
Change-Id: I3cd0e9301c576572194618eba8dc00db06c3c35f