Add flag to have system server wait for a debugger to attach

Sometimes you want to step through something early in system server
startup. For an app, you can run am set-debug-app, but nothing similar
exists yet for system server. This adds a system property that behaves
similarly.

The check for VMDebug.isDebuggingEnabled should ensure this doesn't
trigger in user builds where system server is not debuggable.

Test: boot normally
Test: boot, set property, stop && start, system server waits for debugger
Flag: EXEMPT DEBUG
Change-Id: Ida56935cf9069360a16e9053e573fccbfd85e58b