releasetools: correct allowed property sources for incremental OTAs

When loading build info from a previous version of Android, the set of
allowed property sources should match those available in that version.
In this particular case, the product_services partition was a valid
property source in Android 10.

Bug: 155053195
Test: ran unit tests from test_common.py
Test: generated an incremental OTA which previously failed
Change-Id: Ic0b0a112656533eca78dee31517deff7e3c8d7cc
Merged-In: Ic0b0a112656533eca78dee31517deff7e3c8d7cc