Move initialization of |incremental_apk_exists|

Initialize it regardless of state so that either of the if blocks below
can access it and be safe in doing so.

Bug: 856384
Change-Id: I2205a3351e0ff0603ccaafd0594aaaf9ca8e29ec
Reviewed-on: https://chromium-review.googlesource.com/1118911Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571522}

build/android/apk_operations.py

@@ -844,9 +844,9 @@ class _Command(object):
         if not args.all and not args.devices:
           self._parser.error(_GenerateMissingAllFlagMessage(devices))
 
-      if self.supports_incremental:
-        incremental_apk_exists = False
+      incremental_apk_exists = False
 
+      if self.supports_incremental:
         if args.incremental_json:
           with open(args.incremental_json) as f:
             install_dict = json.load(f)