gdb: add setter / getter for thread_info resumed state

A following patch will want to do things when a thread's resumed state
changes.  Make the `resumed` field private (renamed to `m_resumed`) and
add a getter and a setter for it.  The following patch in question will
therefore be able to add some code to the setter.

Change-Id: I360c48cc55a036503174313261ce4e757d795319