Set WallpaperData flag in constructor

This mWhich field was really meant to be set in the WallpaperData
constructor.

Otherwise, calling bindWallpaperComponentLocked(new
WallpaperData(..., FLAG_LOCK)) will actually try to set the wallpaper on
system only (since the which flag will be 0), causing all sorts of
problems. This happens when trying to temporarily bind a fallback
wallpaper after a reboot, in case the home+lock wallpaper is not
direct-boot aware.

Take this opportunity to remove the old WallpaperData constructor
(this does not change any logic, and the old constructor was only used
in a test), and add a test for the new constructor.

Bug: 286197596
Test: manually verify that the bug is fixed
Test: atest WallpaperManagerTest
Change-Id: Ic55a1d894134405bb325aba04eb42aaca9a1a318