Allow uid to be passed for more operations

This expands get, getmtime, exportKey, getKeyCharacteristcs and begin to
accept a uid to run as. This is only for system to use keys owned by
Wifi and VPN, and not something that can be used to do operations as
another arbitrary application.

Bug: 23978113
Change-Id: I06aa089859edc934a5415e3b184b917d6d171ae2