Initial Secretkeeper HAL service for Trusty

Disabled by default; enable with `export SECRETKEEPER_ENABLED=y` before
building.

Also needs the Secretkeeper TA to be present in Trusty; if the TA is
absent, the HAL service will (repeatedly) fail to connect.

Test: build, VtsSecretkeeperTargetTest
Bug: 306364873
Change-Id: I529013395d0e3afbff4a24b663088adce2a23805