[31/n] Introduce source for rechability to easily fix tests

Current tests implementations consider double-tap for surfaces in
Core. The new logic in Shell is covered with specific tests. To
keep the existing tests unchanged, a ReachabilitySource is
introduced to define a specific code path for the Core case.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 407731267
Test: atest WmTests:AppCompatReachabilityPolicyTest

Change-Id: I114afb8583c4d871fb9070fa768bdba986b53fd4