releasetools: common.LoadInfoDict() takes a bool parameter for repacking.

It used to accept a non-None input_dir to indicate the need to
re-generate images (called from add_img_to_target_files.py). During that
flow, both of the two parameters redundantly refer to the same input
dir. This CL replaces the second parameter with a bool value instead.

Test: `python -m unittest test_common`
Test: `m dist` with aosp_taimen-userdebug
Test: `zip -d aosp_taimen-target_files-eng.zip IMAGES/\* &&
       add_img_to_target_files.py -a aosp_taimen-target_files-eng.zip`
Change-Id: I0a5a164366acb116407f94bb350872a3a0b207d1