liblp: Refactor the partition table update API.

This change separates the WritePartitionTable function into two separate
functions: FlashPartitionTable and UpdatePartitionTable. This will be
cleaner than having extra unnecessary parameters and makes it clearer
when to call each one.

The motivation for this change is to improve the reliability of
UpdatePartitionTable() and to be able to simulate I/O failures for
testing.

Bug: 79173901
Test: liblp_test gtest
Change-Id: Iee5eb2e99fb76aef0b93a65bf85a89907e7cd9bf