Adds Zygote policy flags to control how applications are launched

This patch adds a zygote policy parameter to functions in the
ActivityManagerService (and associated classes) to allow for different
launch behavior based on the triggering event.  This will allow for
latency sensitive applications to utilize the Unspecialized App Process
Pool, without other processes draining the pool and causing excessive
refill events.

Test: Booted and launched several apps
Bug: 147613193
Bug: 146614089
Change-Id: Ib0a51c8720f95a7eafcab4ad2b5bb08f27a58d8c
Merged-In: Ib0a51c8720f95a7eafcab4ad2b5bb08f27a58d8c
(cherry picked from commit 4d051885)