base

Fixing bugs with peeking a wallpaper.

1. Cached wallpaper from one user could be retrieved
for another user. Fixed with adding a mCachedWallpaperUserId
member.

2. mDefault wallpaper was returned too early. Moved down
so that default wallpaper could be returned only after
making sure that system wallpaper is unspecified.

Bug: 25185253
Change-Id: Ie620621e5c4039d5d680211150aa5327f321f11a