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

tools/releasetools/test_sign_apex.py

@@ -35,6 +35,7 @@ class SignApexTest(test_utils.ReleaseToolsTestCase):
     payload_key = os.path.join(self.testdata_dir, 'testkey_RSA4096.key')
     container_key = os.path.join(self.testdata_dir, 'testkey')
     signed_foo_apex = sign_apex.SignApexFile(
+        'avbtool',
         foo_apex,
         payload_key,
         container_key)