Regularize some wallpaper APIs

Specifically:

  + support clearing wallpapers, with similar permissions to setting
  + support adjusting padding, with similar permissions as defining
    wallpaper dimension hints (which behave somewhat similarly)

Bug: 62343054
Test: atest CtsPermissionTestCases:NoWallpaperPermissionsTest
Change-Id: Ia25f2791a20564c58096a25e6e189aa3b06b411c