Throw exception when SyntheticPassword key is missing

Throws early if keystore returns a null key. Otherwise the null
value will be passed into Conscrypt which will throw a confusing
exception "Only SecretKey is supported"

Also add a log when key is deleted on purpose, just in case it
helps us catching the case when the key's gone missing on some devices.

Bug: 141565253
Test: builds
Change-Id: Iee56c8295f84f662876704ac6583f4b34b56876b