releasetools: Prefer the avbtool specified in target_files.

This allows a consistent logic in using the avbtool which could be
board-specific.

Test: `atest releasetools_test`
Test: Run sign_target_files_apks.py on a target_files.zip.
Change-Id: I8cd93b8e71146985734f85c31f4662f5e2e9534c
diff --git a/tools/releasetools/validate_target_files.py b/tools/releasetools/validate_target_files.py
index 5d99c99..435e7f2 100755
--- a/tools/releasetools/validate_target_files.py
+++ b/tools/releasetools/validate_target_files.py
@@ -324,7 +324,8 @@
 
     # avbtool verifies all the images that have descriptors listed in vbmeta.
     image = os.path.join(input_tmp, 'IMAGES', 'vbmeta.img')
-    cmd = ['avbtool', 'verify_image', '--image', image, '--key', key]
+    cmd = [info_dict['avb_avbtool'], 'verify_image', '--image', image,
+           '--key', key]
 
     # Append the args for chained partitions if any.
     for partition in common.AVB_PARTITIONS + common.AVB_VBMETA_PARTITIONS: