Add context for ro.product.property_source_order

Any partitions should be able to write this property with build.prop.
This adds a new context for ro.product.property_source_order so it can
be set from any build.prop, e.g. vendor/build.prop, product/build.prop,
etc.

Bug: 172459064
Test: PRODUCT_VENDOR_PROPERTIES can set this property
Change-Id: Ibf85a4ad02d8454f621428b271e8e298067aa126
4 files changed