ANDROID: trace: add non-hierarchical function_graph option

Add the 'funcgraph-flat' option to the function_graph tracer to use the default
trace printing format rather than the hierarchical formatting normally used.

Change-Id: If2900bfb86e6f8f51379f56da4f6fabafa630909
Signed-off-by: Jamie Gennis <jgennis@google.com>