init: setup keyring before ueventd starts

Invent keyutils.h to supply capability to set session keyring.
The keyring will hold things like the FBE encryption keys.

Test: gTest logd-unit-tests --gtest_filter=logd.statistics
Bug: 37751120
Bug: 36645158
Change-Id: Ieb44fa8f53dda6cf506a6243498c72d7f7f3cde7