Look for non-existent files listed in avb_vbmeta_args.

In BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS, if we have defined
"--include_descriptors_from_image" with an image file whose path points
to source tree, add_img_to_target_files.py or sign_target_files_apks.py
may fail to find the file. Because these scripts may run without a
source tree, by taking target_files.zip as the only input.

This CL scans additional locations in the input target_files.zip to find
those missing files in avb_vbmeta_args. As long as the files are included
in the target_files.zip, they get a second chance to be found.

Bug: 63910867
Test: As follows:
 1. Setup BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS with a local file path;
 2. Remove the local file;
 3. sign_target_files_apks.py fails without this CL;
 4. sign_target_files_apks.py works.
Change-Id: I3c58f80a5535db02b74cfe40d0c0beff72587cf8