Themes: Make notification list bg themeable

Thanks Thomas Ekelschot :)

Screen of themed resource https://imgur.com/c8t1Zn0

PS2: Add Clark's proposed changes
PS3: Cleanup old color resource in main xml
PS4: Add informative screen

Change-Id: I8eb4427747c3eb035b416860c3ca47122e45c16f
IconPackHelper: Don't respect an icons original size if too large.

  - Otherwise, if an application isn't following app icon guidelines,
  it can create an OOM situation when creating the new icon based
  on the original resource's height and width.

Change-Id: I9430445da5e393cdbf76ed9ebd9698bf30df6a95
Themes: Add STYLE_THUMBNAIL to PreviewColumns

Change-Id: Ie18264cbede92220335400428e67735866cd442d