Do not assign 'false' to PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE

When OVERRIDE_PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE is set to
false, it must not copy the value to
PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE but leave it empty
because Soong always set EnforceProductPartitionInterface to true if
PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE has value.

Bug: 148952638
Test: Build with
      OVERRIDE_PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE=false

Change-Id: I6435157ff993e53669b4244c9be74efbcc84107d