libselinux android: Add keystore2_key label module.

This patch adds a new Android specific module for keystore2 key
namespace lookup. It assumes the presence of keystore_contexts files
mapping numeric namespace identifiers to SELinux labels. Keystore 2.0
uses this module to exert more granular access control and allow system
and vendor components to share explicit key namespaces.

Bug: 158500146
Test: N/A
Change-Id: Ic6c186cb4c2fb1e6e5ddd8494e89f72cfb3119ba
9 files changed