Install (vendor|system/vendor)/build.prop always

For now, vendor build properties are added into /system/build.prop
when property split isn't enabled.
So we have the duplicate codes to add them to /system/build.prop and
/vendor/build.prop case by case.

But either /vendor/build.prop or /system/vendor/build.prop can exist
always.
So this CL will install $(TARGET_OUT_VENDOR)/build.prop always and
remove the duplication.

Bug: 68115808
Test: tested on aosp_x86-userdebug
Change-Id: Ic734418890629d011c733c2d8d14739275e64a78