releasetools: Rename the ramdisk_dir in prop dict to root_dir.

This property indicates the path to the root directory (i.e.
TARGET_ROOT_OUT at build time, or ROOT/ in a target_files.zip).
build_image.py will only use this property when building a target using
system_root_image. Under such a configuration, files under this dir
never land into a ramdisk, but as part of system.img. This CL renames
the property name to avoid confusion.

Note that with this change releasetools keeps working with older
target_files zips that contain 'ramdisk_dir'. Because common.py doesn't
read the value of that property, but sets it to ROOT/ directly.

Test: `m dist` on aosp_marlin-userdebug.
Test: `m dist` on aosp_angler-userdebug.
Test: Run add_img_to_target_files.py with a new target_files.zip and an
      old target_files.zip respectively.
Change-Id: Ie9e19dd17d8c2abb4c4a97449e436774d5dd2a36