liblp: Allow flashing to readonly logical partitions.

By default, logical partitions with the readonly flag are created with a
readonly device. This change allows callers of CreateLogicalPartition
to create writable devices so they can still be flashed.

Bug: 78793464
Test: fastboot flash product_services works with fastbootd
Change-Id: Ia8d2761a3067e3b62815acbf0b6fb7f033072ba2