releasetools: Support OTAs that have OEM properties changes.

We need to handle a special case that an OTA goes from a source build
without OEM properties to a target build with those properties (or vice
versa). Add support in OTA scripts to deal the case properly, by a)
using two oem_props variables to handle source and target builds
respectively; b) adjusting the fingerprint/thumbprint assertions to
allow a mix of both.

Change-Id: Ib517c366730b967a974c89528df9d42887c92ec2