Initialize |incremental_apk_exists|

Fixes a script crash from |incremental_apk_exists| being used without
being initialized.

NOTRY=true # bot failures unrelated

Bug: 856384
Change-Id: Iea429a58610bc9d409f35ee6b5cf3314bb12eab7
Reviewed-on: https://chromium-review.googlesource.com/1114329
Commit-Queue: agrieve <agrieve@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570306}

build/android/apk_operations.py

@@ -845,6 +845,8 @@ class _Command(object):
           self._parser.error(_GenerateMissingAllFlagMessage(devices))
 
       if self.supports_incremental:
+        incremental_apk_exists = False
+
         if args.incremental_json:
           with open(args.incremental_json) as f:
             install_dict = json.load(f)