releasetools: Move the AVB salt setup into common.LoadInfoDict().

We used to do this in add_img_to_target_files.AddImagesToTargetFiles(),
which didn't cover the path when calling from make_recovery_patch. As a
result, /system/bin/install-recovery.sh contains different SHA values
from the actual images.

Test: Set up aosp_bullhead to use AVB. `m dist`, then run the following
      command to verify the generated install-recovery.sh.

  $ ./build/make/tools/releasetools/validate_target_files.py \
      out/dist/aosp_bullhead-target_files-eng.zip

Change-Id: Id7be8fb17072252fcd4d08db2057b8c4af053376