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
9a13f36cdd