Add transaction executor

This adds TransactionExecutor class, that takes care of executing
a multi-stage ActivityManager client transaction in correct order.

First it executes all callbacks, while also making sure to transition
to the right pre- and post-execution state if requested.
In the end it cycles to the final requested lifecycle state.

This also switches activity launch process to use lifecycler - it
initializes activity launch and sets final desired state in the same
transaction.

Bug: 64797980
Test: android.app.servertransaction.TransactionExecutorTests
Change-Id: I306f9396fab263682f580cc8c924a3cedb40ef89