[automerger skipped] Stop relying on background color attribute to set task...

[automerger skipped] Stop relying on background color attribute to set task transition background am: a5f3e902 -s ours

am skip reason: Merged-In Id42f52760cb9a681c6c33f3c261d7db5e2f71af3 with SHA-1 c26db36f is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/15897416

Change-Id: I912a9b3ecd3e318864adea07f3b064f01b2df22c