releasetools: Update the handling of recovery fstab.

First, remove the use of info_dict['fstab'] in add_img_to_target_files.
 - info_dict['fstab'] corresponds to recovery fstab
   (`/etc/recovery.fstab`), which may differ from the one used for
   normal boot.
 - When calling build_image.ImagePropFromGlobalDict, we already have the
   desired info from global dict (`META/info_dict.txt`).

Second, common.LoadInfoDict now loads recovery fstab only for non-A/B
devices. Because the info is only meaningful for installing non-A/B OTAs
(under recovery mode).

Fixes: 132458722
Test: TreeHugger
Test: Build non-A/B incremental OTAs.
Change-Id: Id23e7b17264c123319fe00b1663d52bfd9b4a5e2