Respect Drawable's alpha when drawing UserBadgeDrawable's background

- Before this fix, the badge's background is always solid even when Drawable's alpha is 0. This caused a white dot to appear in Overview for the splash icon.
- Applied alpha to the paint when drawing UserBadgeDrawable's background

Fix: 316576459
Test: Quick switch / Swipe down with out of orientation thumbnail show splash correctly
Test: Rotate in Overview doens't cause white dot to appear for work profile icons
Test: UserBadgeDrawableTest
Flag: EXEMPT bugfix
Change-Id: I8b8e52aba12a1c7d048a9b9d4dd2a556f6788eba