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
Change-Id: Ib0a51c8720f95a7eafcab4ad2b5bb08f27a58d8c