Move bitmapSupplier.get() call to a worker thread.

bitmapSupplier.get() is called twice which is unnecessary.
Call bitmapSupplier.get() in the constructor instead.

Bug: 232850298
Test: Manual.
OverView - shareImageToTarget https://b.corp.google.com/issues/232850298#comment8
Screenshot share from Toast - https://b.corp.google.com/issues/232850298#comment7

Change-Id: I920f89b90fafcc0d51b1b948fd76fdc9aaee4c52