releasetools: Address pylint warnings in add_img_to_target_files.py.

C:661, 0: Wrong hanging indentation (add 2 spaces).
        OPTIONS.input_tmp, "VENDOR_IMAGES")
        ^ | (bad-continuation)
C:673, 0: Wrong continued indentation (add 14 spaces).
                os.path.join("IMAGES", img_name))
                ^             | (bad-continuation)
C: 54, 0: Import "import datetime" should be placed at the top of the module (wrong-import-position)
C: 55, 0: Import "import hashlib" should be placed at the top of the module (wrong-import-position)
C: 56, 0: Import "import os" should be placed at the top of the module (wrong-import-position)
C: 57, 0: Import "import shlex" should be placed at the top of the module (wrong-import-position)
C: 58, 0: Import "import shutil" should be placed at the top of the module (wrong-import-position)
C: 59, 0: Import "import subprocess" should be placed at the top of the module (wrong-import-position)
C: 60, 0: Import "import tempfile" should be placed at the top of the module (wrong-import-position)
C: 61, 0: Import "import uuid" should be placed at the top of the module (wrong-import-position)
C: 62, 0: Import "import zipfile" should be placed at the top of the module (wrong-import-position)
C: 64, 0: Import "import build_image" should be placed at the top of the module (wrong-import-position)
C: 65, 0: Import "import common" should be placed at the top of the module (wrong-import-position)
C: 66, 0: Import "import rangelib" should be placed at the top of the module (wrong-import-position)
C: 67, 0: Import "import sparse_img" should be placed at the top of the module (wrong-import-position)
W:391,12: Redefining built-in 'dir' (redefined-builtin)

After this CL, it still gives the warning below, which is to be
addressed with follow-ups.

R:635, 2: Too many nested blocks (6/5) (too-many-nested-blocks)

Test: `m dist`
Test: pylint --rcfile=pylintrc add_img_to_target_files.py
Change-Id: I64da184b6b69e93449dbfc989a5d7f46d5223f42