Rate limit notifyColorsChanged()

Throttle onComputeColors to make sure that a wallpaper
won't affect general system health easily.

Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/wallpaper/WallpaperServiceTests.java
Fixes: 63585249
Change-Id: Ib4317c0b4d284d4cd4a640c24ec71b272b8493d0