Load video thumbnails using ARGB_8888 for quality.

Based on feedback from a partner, it sounds like getFrameAtTime()
and getScaledFrameAtTime() may end up returning low-quality bitmaps.

To make it clear that we want to create high-quality thumbnails,
this change explicitly requests ARGB_8888.

Bug: 157448043
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I39c7c552e8fa715ec13efa75c1c3a30d58492bfb