Refactor private bitmap methods to handle `which` correctly

Changes the private peek methods in WallpaperManager.Globals to accept
and/or properly handle a `which` parameter. Also adds an override with
`which` to the hidden public method peekBitmapDimensions().

Bug: 253481667
Test: TreeHugger
Change-Id: I49e7b816272dc079bf82030d81cb7a0870defa62