protozero: Expose bool op + get() on MessageHandle
protozero: Expose bool op + get() on MessageHandle

The bool operator was previously hidden behind MessageHandleBase - and
the get() method is useful e.g. if you want to pass a pointer of to the
pbzero object to a helper function.

Change-Id: I33a99da8acfb7e47415dabee0c15ed99a1ba9712

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto