Mark threads started by Daemons.java as systemDaemon

The threads started by Daemons.java are special daemon threads that
provide important GC related functionality to the runtime. As such the
runtime sometimes needs to keep track of which threads these are. This
adds support for keeping track of that data.

Also give a way for the runtime to wait for all Daemon threads to come
up.

Test: while atest CtsJdwpTestCases; do; done;
Exempt-From-Owner-Approval: Previous version approved by nfuller@ with
                            only minor nit changes. Bypassing further
                            owner review since all owners are currently
                            in LON.
Bug: 123696564
Change-Id: Ia70da0b4d11409930bef47e4d06359b0d612c8fa
2 files changed