Fix signapk not found error when running ota_from_target_files locally

location of ota_from_target_files changed from
out/host/linux-x86/bin to
out/soong/host/linux-x86/bin . This changes relative position of
signapj.jar. To fix, use ANDROID_HOST_OUT as search path

Change-Id: I5397171566e9d7598b5ef16ae26641f0c183d748
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index e99152a..5d9cbd2 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -1479,7 +1479,7 @@
     # Only check for existence of key file if using the default signer.
     # Because the custom signer might not need the key file AT all.
     # b/191704641
-    if not OPTIONS.signapk_path:
+    if not OPTIONS.payload_signer:
       private_key_path = OPTIONS.package_key + OPTIONS.private_key_suffix
       if not os.path.exists(private_key_path):
         raise common.ExternalError(
@@ -1487,6 +1487,11 @@
             " correct key path through -k option".format(
                 private_key_path)
         )
+      signapk_abs_path = os.path.join(
+          OPTIONS.search_path, OPTIONS.signapk_path)
+      if not os.path.exists(signapk_abs_path):
+        raise common.ExternalError(
+            "Failed to find sign apk binary {} in search path {}. Make sure the correct search path is passed via -p".format(OPTIONS.signapk_path, OPTIONS.search_path))
 
   if OPTIONS.source_info_dict:
     source_build_prop = OPTIONS.source_info_dict["build.prop"]