Make tiered cached adj UI tier size configurable

This change makes the UI tier size configurable through overlays. By
default, the UI tier can contain 10 processes.

Test: - atest MockingOomAdjusterTests
      - device_config put activity_manager
        tiered_cached_adj_ui_tier_size, followed by dumpsys activity
        settings to check that the size was updated
      - build with a device-specific config to check that the size is
        non-default
Bug: 373863708
Bug: 374541382
Flag: com.android.server.am.oomadjuster_cached_app_tiers
Change-Id: Iadca88f79c232a8fb5f6a5a69807a30029bb8a8c