Deprecate ObjectPool and ObjectPoolItem (1/n).

Begin deprecating the ObjectPool class and ObjectPoolItem interface.
Direct object instantiation is now preferred over using the pool.
This change is the first in a series of changes to remove the
object pool mechanism and simplifying object management.

Bug: 311089192
Test: atest FrameworksCoreTests:ObjectPoolTests
Flag: EXEMPT removing com.android.window.flags.disable_object_pool
Change-Id: If04abad2263be437691b99c4ed166b91dcab3a7f