Better systrace slices for application startup.

This change adds three new timeslices :

- PostFork : As soon as possible after the app forks from the
  zygote. Can be used in conjunction with the system_server
  "Start proc:" event to derive an upper bound on fork() and
  zygote overhead.

- RuntimeInit & ActivityThreadMain for ZygoteInit#runtimeInit
  and ActivityThread#main.

ActivityThread#handleBindApplication and higher level functions
are already well instrumented in systrace. handleBindApplication
should occur immediately after ActivityThread#main.

Note that we use the Activity manager tag to make it easier to
correlate these new events with surrounding events (Start proc
and handleBindApplication) that are already using the AM tag.

bug: 21632700
Change-Id: Ibc01f1721f962c913f3c02a51763b6feb1eb6a4d