Assume the god of chance is not with you
I have been getting armour from my zombie converter, now at a more PC-esque rate
iOS iPad mini gen 2
iOS 10
0.16.0
Mobs aren't supposed to drop armor every time; zombie drop rate is 8.5% for naturally spawned equipment
Refer to MCPE-9846
Mobs dropping armour every time was a bug, but is now fixed
Mending and Frost Walker enchantments do not yet exist in MCPE
Edit: I did not read the version before reading the report
It appears, according to the minecraft wiki, that while the enchantments for Mending and Frost Walker were added in 1.1 build 1, the book form was not. Although I do not have the beta build to check creative mode for the book, that is what the MC wiki appears to imply in the 1.1 build 1 page.