Envoy prefs-based key-value store

both https://github.com/envoyproxy/envoy-mobile/issues/1587 and https://github.com/envoyproxy/envoy-mobile/issues/1587 have limited utility while they only cache to the in-memory key value store.

Upstream we have a file based key-value store (https://github.com/envoyproxy/envoy/blob/main/source/extensions/key_value/file_based/config.cc) but for android we need
1) an API to write strings to and read strings from the android prefs file and
2) a key value store which uses that API to load and flush from prefs

We've discussed this one at the weekly meeting, but there was no tracking bug so filing one now :-)