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/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 668ad9b..5d0efc3 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -468,6 +468,7 @@
             maxsize, name, payload_key))
 
         signed_apex = apex_utils.SignApex(
+            misc_info['avb_avbtool'],
             data,
             payload_key,
             container_key,
@@ -572,7 +573,8 @@
       # key is specified via --avb_system_other_key.
       signing_key = OPTIONS.avb_keys.get("system_other")
       if signing_key:
-        public_key = common.ExtractAvbPublicKey(signing_key)
+        public_key = common.ExtractAvbPublicKey(
+            misc_info['avb_avbtool'], signing_key)
         print("    Rewriting AVB public key of system_other in /product")
         common.ZipWrite(output_tf_zip, public_key, filename)