Fix: Improve vis oom scores in split screen and desktop windowing.

Previously, visible applications in desktop windowing mode received a
`vis` oom score, with their Z-index (`vis+X`) added to prioritize
foreground apps. This was effective for single-window, single-process
applications, making those above others less likely to be targeted by
the Low Memory Killer Daemon (lmkd).

However, this approach fell short for multi-window and multi-process
applications (e.g., browsers). Without a direct mapping between an app's
activities and its various processes, assigning a correct `vis+X` oom
score to unfocused windows of an app with a focused window was
impossible. This led to unexpected and suboptimal killing behavior.

This change addresses the issue by laddering the oom scores of processes
within the `vis` range based on their relative positions in the process
LRU (Least Recently Used) list. The most recently used process now
receives a `vis+0` score, and subsequent processes receive `vis+X`
(where X is their position in the LRU). This leverages the existing
mechanism where processes are moved to the top of the LRU list when
focused, and allows applications to modify binding flags based on
`onWindowFocusChanged` events, ensuring consistent and correct oom score
assignments across all processes of a multi-window app.

Bug: 421872956
Test: atest MockingOomAdjusterTests
Flag: com.android.server.am.oomadjuster_vis_laddering
Flag: com.android.server.am.remove_lru_spam_prevention
Change-Id: Ie5436154bdd7423501ceb8f7d2118e6fdcfb0a61