[rvc] Define vendor-specific property ro.incremental.enable

[already merged in master and AOSP]

Make ro.incremental.enable a vendor-specific property. Allow
system_server and vold to read this property.

Test: manual
BUG: 155212902
Merged-In: I8ff8837af635fa8e7b5bb02e5f6de5ac15b5023b
Change-Id: Id432390023de232deb4cc4d0ff3fb73904093b60
12 files changed