Apply a new icon cache mechanism for memory improvement

- Avoid loading all app icons at once to decrease memory usage.
- Only load visible icons when entering the apps page.
- Reserve icon placeholder to alleviate icon loading flicker.
- Release icon cache when low memory or leaving apps page.

Bug: 187118427
Bug: 209898662
Test: manual check the smoothness and memory usage of apps pages.
Change-Id: Ifc3c2a73cc88d6e42739df4e8208445afa12e0ea