Create frozen flag for OomAdjuster rewrite

The `oomadjuster_correctness_rewrite` is overridable by setting system
property `persist.sys.activity_manager_native_boot.enable_new_oom_adj`
and a reboot (for local testing only!).

(ex.
to force new algorithm:
`setprop persist.sys.activity_manager_native_boot.enable_new_oom_adj true`
to force old algorithm:
`setprop persist.sys.activity_manager_native_boot.enable_new_oom_adj false`
to revert to build default:
`setprop persist.sys.activity_manager_native_boot.enable_new_oom_adj \"\"`
)

Test: n/a
Bug: 298055811
Change-Id: Ib5d985bd1ed229889a6b6bbe4f5b5191ff7b7d57