Loose the translucent condition when estimate splash screen bg color.

Only use the System background color if more than half of pixels in the
window background drawable are translucent. Increase the tolerance of
gradient drawable object.

Bug: 294505626
Test: cold launch sample app, verify splash screen draw with dominate
color.

Change-Id: I46c88a1bf2c5fb7d5152d147f035bb172b3f9f78