Fix Add button animations

Use AnimatorSet to make the fade animations sequential instead of
simultaneous. Also reduce the total duration to 200ms (100ms fade out,
100ms fade).

Before: http://screencast/cast/NjQ4NTcwMjcyNzQzNDI0MHxjNDY5NDJiNy03Mg
After: http://screencast/cast/NTg5NDA0NDI5ODI0ODE5MnxmMjczMTlmYi05Mw

Bug: 333734220
Test: manual, see screencast
Flag: ACONFIG com.android.launcher3.enable_widget_tap_to_add NEXTFOOD

Change-Id: I2ee87f33c46e5dd07974152fa32393c39a18ecf1