liblp: FlashPartitionTable should update all slots.

Logical partition metadata has "slots" for AB purposes, but when
flashing or updating the partition table via fastboot, there is no
reason not to synchronize all copies of the metadata. It makes the state
of the super partition much more clear. It also makes the super
partition less likely to break on a slot change from the user, for
example if a "_b" partition is created before changing to the "b" slot.

Bug: 78793464
Test: liblp_test gtest
Change-Id: I3c44f0362f21f87d0bfc3a5c3394e26dc3dd38be