Sets system content priority based on interaction.

Keep track of the input focus changes and assign high system content
priority to windows that are recently interacted. When a new window
gained focus, it assumes that the window is recently interacted.

Bug: 431285755
Test: unit tests
Flag: com.android.window.flags.system_content_priority

Change-Id: Ia1b10eb0b8f1ac58b046951010fccabbc2c904be