Disable ObjectPool in WM server transaction w/ flag.

ObjectPool is bypassed if the flag is enabled, letting the GC handle
lifecycle items. This addresses unnecessary overhead from synchronized
map operations and allows using the 'final' keyword on pool items for
further cleanup.

Benchmarking each test case (50,000 iterations) shows
~25% (193.77 ms) speedup with ~15% GC time (9.49 ms) tradeoff:
http://gpaste/5510735790866432

Bug: 311089192
Test: atest FrameworksCoreTests:ObjectPoolTests
Flag: com.android.window.flags.disable_object_pool
Change-Id: I1eb5de46e7b58ab5356bcdd4d9bb0c2fcf99efeb