JDWP: more tests for StackFrame.SetValue

Tests we correctly set values in the following contexts:
- "constant" local variable
- exception handling (catch and unwind)
- native / proxy transition

Also changes the way we suspend the debuggee. We now synchronize
through a socket to suspend the debuggee instead of suspending it
on a breakpoint.

Bug: 19944235
Change-Id: I88432133aec4225f5ce079086b60c49a209043c7