Replace WallpaperData File fields with methods

WallpaperData objects do not need to store File objects directly. There
are 4 wallpaper files per user, and since File objects are immutable in
java we can keep a global reference of them. WallpaperData objects can
retrieve the right files using their mWhich and userId fields.

Doing so prevents ending up in weird states, e.g. if the mWhich flag
changes but not the wallpaperFile or cropFile.

Test: atest WallpaperManagerTest
Test: atest WallpaperManagerServiceTests
Bug: 286909680
Change-Id: I4cf9fc6311238d94213c4af3c6fb8beed9fc9023