Update docs for Keystore audit events

The audit events emitted by `keystore2` since Android S use the top bit
of the owner field to indicate whether the key is associated with an app
or an SELinux domain.  Update the docs to reflect this behaviour.

Bug: 438126526
Bug: 431208221
Test: examined javadoc
Flag: EXEMPT doc update
Change-Id: I8e71651eed2c83196ca28667b2d4d3fd4d5020e4