Update ImageDecoder docs regarding Files

Bug: 139186570
Test: make docs

As part of enforcing Scoped Storage for the next Android release, we're
encouraging app developers to steer away from using raw file path access
and, instead, use FileDescriptor or Uri to access files.

Update createSource(File)'s docs to suggest why they might want to use a
different method.

Update the top level example to use createSource(ContentResolver, Uri)
instead of createSource(File).

Change-Id: Icbccdb5183edd0b6a052ec905f999f02ac4e61d3