DO NOT MERGE: Don't do heavy operation while in the synchronized block

We read the wallpaper within the synchronized block, this is a long time
operation, might block the other thread who is going to obtain the lock.
So we have to move the operation beyond the synchronized block.

Bug: 228435216
Test: atest WallpaperManagerTest#testPeekWallpaperNotBlockingOtherThread
Change-Id: Ic2a947b473710ffe68db436586769858950588a4