releasetools: Make apex_utils Python 3 compatible.

In order to get a Python 2 and 3 compatible re-raise behavior, this CL
removes the stack traceback for the lines within apex_util module (i.e.
sys.exc_info()[2]). It's not a big loss in practice, since we only have
one line within the try-except block (`common.RunAndCheckOutput()`)
that's no longer reported in the traceback.

Using `six` module could better solve this, but only after building
releasetools as python_binary_host modules where we can properly handle
the module dependency.

Bug: 131631303
Test: TreeHugger
Test: `python -m unittest test_apex_utils`
Test: `python3 -m unittest test_apex_utils`
Change-Id: I0c5a72ec9fad5ff9d8c9c94d29e813e433ec2921