Fix access to FP registers when visiting stack

Adds GetFPR and SetFPR to Context class so we can read from and write to
floating-point registers during stack visit. They return a boolean flag
indicating whether the read/write is successful. This allows the debugger to
return the JDWP error ABSENT_INFORMATION when we can't read/write a register.
We also update GetGPR and SetGPR for consistency. We keep a default GetGPR
implementation asserting the read was successful using a CHECK so we don't
silently fail.

Adds missing JDWP object tags for StackFrame.SetValues to avoid crash when
setting corresponding objects (thread, thread group, class object or class
loader). Also returns JDWP error INVALID_OBJECT (when the given object id is
invalid) instead of crashing with an unimplemented message.

Bug: 15433097
Change-Id: I70843c9280e694aec1eae5cf6f2dc155cb9ea10e