Workaround wallpaper restore race

If WallpaperManager is asked to read a wallpaper between when we
call mWm.clear and mWm.setStream, it is possible for us to end
up losing restored state such as cropHint and allowBackup

By not calling mWm.clear, we make this much less likely to happen.
It isn't necessary to call clear, as will explicitly overwrite
all the interesting state anyway.

Test: by repeated adb restore
Bug: 34760753
Change-Id: I2750695d5dd16077b47b5b8fca3a667b98cc4f6c