Keystore 2.0: Add run_as to keystore2_test_utils

The run_as function allows a test with sufficient privileges to run a
closure as different identity given by a tuple of UID, GID, and SELinux
context. This is infrastructure in preparation for the keystore2 vts
test.

Test: keystore2_test_utils_test
Bug: 182508302
Change-Id: Ic1923028e5bc4ca4b1112e34669d52687450fd14
6 files changed