64-bit GSI: adds init.zygote32_64.rc into root dir for vendor to select

For 64-bit GSI, there are two different zygote settings:
  1. 64-bit primary, 32-bit secondary (init.zygote64_32.rc) OR
  2. 32-bit primary, 64-bit secondary (init.zygote32_64.rc)
  3. 64-bit only is currently forbidden (b/64280459#comment6)

This change adds the second setting into root dir for vendor.img to
select. The first setting already exists.

PRODUCT_DEFAULT_PROPERTY_OVERRIDES += ro.zygote=zygote64_32 is removed
because the property will be put into vendor.img. i.e., it has no effect
on GSI.

Bug: 64280459
Test: build and boot device, checks init.zygote32_64.rc is in rootdir
Change-Id: I16b36018acac8cfeee7281d6f4b6c9c19467abec