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