docs: Added curly brace in sample code method opening.

Added a curly brace at the beginning of "onProvideShadowMetrics()."
This method appears within the sample code associated with step 2 of
the "Designing a Drag and Drop Operation" section of the topic.

Bug: 24561657
Change-Id: Icb06a65f63bc57c747bb6f244f6b623c17bdfc45