Enables the use of the blastula pool.

This commit adds the code necessar to initialize and use the blastula
pool during application launching.  Highlights include:
* Modifying ZygoteState to allow the creation of blastula session
sockets
* Modified application startup to track if a web view process is being
created.
* Initialization of the blastula pool during Zygote initialization.
* Blastula lifecycle management via reporting pipes and event FDs.
* Launching of applications via the blastula pool.

The creation, maintenance, and use of the blastula pool can be disabled
by setting Zygote.BLASTULA_POOL_ENABLED to false.  When this feature is
disabled applications will launch as they did before this patch.

Topic: zygote-prefork
Test: make & flash & launch app & check log message
Bug: 68253328
Exempt-From-Owner-Approval: No changes to files with owners between revisions
Change-Id: I46c32ad09400591e866b6c6121d5a9b0332092f3