[DO NOT MERGE] Rollup Cert-related changes from R

This change includes the following commits from internal R branch
which are related to certificate management:

0206e76f46 CredentialStorage: Install keys using KeyChain
09ceea53d9 Added functionality to select type of certificate to be installed from the Settings app
3acf3f5433 WiFi certificates installable from Wifi sub-preference
8439fd15f7 Fix strings for certificate installation in Settings

Bug: 161347472
Test: builds & manual testing
Change-Id: Ia59dc4780254fab4f34c2f61b25f3b4e56ed7b77