ro.zygote in base_system.mk is optional

The setting of ro.zygote in base_system.mk is optional, which means the
value can be overriden by other (probably more specific) mk files.

Bug: 117892318
Bug: 158735147
Test: atest --host post_process_prop_unittest
Change-Id: Ia7a67c0a04fad343d6591417f40dd4b9ddadc5e4