Fix adaptive shortcut icons

Use adaptive icon for the static add contact shortcut.
Add O specific shortcut icon size.
Instead of adding padding after already getting the drawable from
the letter avatars, scale the drawable down by the same percentage
to get the padding. This eliminates the extra transparent part around
the icons, which led to the launcher filling in with black.

Test: manually verified on Nexus 5X with O
(https://screenshot.googleplex.com/Pa1t37DqeNJ)
and Pixel with O
(https://screenshot.googleplex.com/qujpyze1Euz)
and Nexus 6P emulator running N
(https://screenshot.googleplex.com/1KWQPx8RZFC)
and Samsung Galaxy S7 running M
(https://screenshot.googleplex.com/J39VES1jT8o)

Bug: 37922561
Change-Id: I6573f91e5441333eda05ec2640dc5137890be317