Fix _ImportParser() error if property is imported from oem partition.

The oem partition allows system build.prop to import properties from it
by "import /oem/oem.prop xxxx". An _ImportParser() error was raised on
this case.

BUG: 154171021
Test: 1) "atest --host releasetools_test releasetools_py3_test -c"
      2) On a device who has oem partition, "make dist" and sign its
      target zip file.
Change-Id: I47875bf7a698390e11690150e6516a3064550ca0
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 7805e30..42c29c0 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -803,8 +803,13 @@
     """Parses the build prop in a given import statement."""
 
     tokens = line.split()
-    if len(tokens) != 2 or tokens[0] != 'import':
+    if tokens[0] != 'import' or (len(tokens) != 2 and len(tokens) != 3) :
       raise ValueError('Unrecognized import statement {}'.format(line))
+
+    if len(tokens) == 3:
+      logger.info("Import %s from %s, skip", tokens[2], tokens[1])
+      return {}
+
     import_path = tokens[1]
     if not re.match(r'^/{}/.*\.prop$'.format(self.partition), import_path):
       raise ValueError('Unrecognized import path {}'.format(line))