Return NetworkStats bundle from NM service.

Introduce NetworkStats which is a collection of network statistics,
which should match the structure communicated by kernel module through
netd.  Will introduce tags and fg/bg stats later.  Kept entirely in a
flat data structure to optimize parcel speed.

Initial pass at returning NetworkStats from NetworkManagementService,
both summary and details.  Will eventually pull data from kernel module
over netd connection.

Change-Id: I92d9f61678ec8c22e2ce26775fb035a0cf32413f