Make DropRootUser more container-friendly

When Android is running as a container, it might not have CAP_SYSLOG
(since all the checks in the kernel are performed against the init
namespace). This change makes DropRootUser not raise CAP_SYSLOG if it
wasn't present to begin with.

Bug: 74568776
Test: DropRootUser no longer returns false when run in a container
Change-Id: Ia54ea81f51cc602c6304a81f50a55b2ccde7df36