JDWP: update thread synchronization

This CL ensures only one thread can do JDWP stuff at a time: either
processing a command coming from the debugger (JDWP thread) or
sending an event (breakpoint, class prepare, etc) to the debugger
before suspending.

The JDWP thread now uses AcquireJdwpTokenForCommand and
ReleaseJdwpTokenForCommand, respectively acquiring and releasing the
token used for synchronization. On the other hand, the event threads
now use AcquireJdwpTokenForEvent and ReleaseJdwpTokenForEvent.

During an invoke, the target thread needs to take the JDWP token to
execute the method while it's already held by the JDWP handler thread
waiting for the invocation to complete. To avoid both threads from
waiting for each other (deadlock), the JDWP thread releases the token
and acquires it again after the invocation is complete, so the target
thread can run safely and prevents other threads from sending events.

Bug: 19120467
Change-Id: Ie3208fb940a60573769d494128cf22f0fa30fa61

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