releasetools: Add a BuildInfo class.

Prior to this CL, we have similar codes in different paths
(WriteFullOTAPackage / WriteBlockIncrementalOTAPackage /
WriteABOTAPackageWithBrilloScript). This CL factors out the common codes
that deal with OEM-specific properties into BuildInfo class, and adds
tests for the new class.

Test: python -m unittest test_ota_from_target_files
Test: Create an incremental package that uses and doesn't use OEM
      properties respectively.
Test: Create a full package that uses and doesn't use OEM properties
      respectively.
Change-Id: Ida914cace12803d55396fa503bafcac2db2a520e
diff --git a/tools/releasetools/edify_generator.py b/tools/releasetools/edify_generator.py
index 0c44faf..7a81928 100644
--- a/tools/releasetools/edify_generator.py
+++ b/tools/releasetools/edify_generator.py
@@ -77,14 +77,14 @@
     with temporary=True) to this one."""
     self.script.extend(other.script)
 
-  def AssertOemProperty(self, name, values):
+  def AssertOemProperty(self, name, values, oem_no_mount):
     """Assert that a property on the OEM paritition matches allowed values."""
     if not name:
       raise ValueError("must specify an OEM property")
     if not values:
       raise ValueError("must specify the OEM value")
-    get_prop_command = None
-    if common.OPTIONS.oem_no_mount:
+
+    if oem_no_mount:
       get_prop_command = 'getprop("%s")' % name
     else:
       get_prop_command = 'file_getprop("/oem/oem.prop", "%s")' % name