Fix error when using -p

args.file is never set after Ieb9a87f16cf5e1c07a65186eff5df26d4ffcdd33,
add a helper function to get the file instead to fix:
Traceback (most recent call last):
  File "./update_current.py", line 576, in <module>
    if args.file:
AttributeError: 'Namespace' object has no attribute 'file'

Test: ./update_current.py -p 4583087
Change-Id: I4fdcf76a267e70f3eb3f3ec8a641983fd91b8357
diff --git a/update_current.py b/update_current.py
index bb80208..d82ce94 100755
--- a/update_current.py
+++ b/update_current.py
@@ -481,6 +481,13 @@
   else:
     raise Exception('Updating this set of prebuilts requires <source> to be a numeric build id, not "' + source + '"')
 
+def getFile(args):
+  source = args.source
+  if not source.isnumeric():
+    return args.source
+  else:
+    raise Exception('Updating this set of prebuilts requires <source> to be file, not a numeric build id')
+
 parser = argparse.ArgumentParser(
     description=('Update current prebuilts'))
 parser.add_argument(
@@ -552,10 +559,7 @@
             print_e('Failed to update platform SDK, aborting...')
             sys.exit(1)
     if args.design:
-        if not args.file:
-            print_e('Design Library must have --file specified')
-            sys.exit(1)
-        elif update_design(args.file):
+        if update_design(getFile(args)):
             components = append(components, 'Design Library')
         else:
             print_e('Failed to update platform SDK, aborting...')
@@ -573,7 +577,7 @@
     subprocess.check_call(['git', 'add', current_path])
     subprocess.check_call(['git', 'add', system_path])
     subprocess.check_call(['git', 'add', buildtools_dir])
-    if args.file:
+    if not args.source.isnumeric():
         src_msg = "local Maven ZIP"
     else:
         src_msg = "build %s" % (getBuildId(args))