Add support for TARGET_PRODUCT_PROP

Like TARGET_SYSTEM_PROP, but is merged into /product/build.prop.

Also change the formatting of the output of both of these macros
slightly, so make the start and end clearer.

Bug: 119911662
Test: migrate downstream products to use this
Change-Id: Iddae7aac3c51b5706f3fb2690c5ca5a98840ad34