Fix image collection vis bug

[giswqs]

This PR fixes the bug reported in #1952 @jdbcode.

When adding an image collection to the map, it should be converted to an image using collection.mosaic(), which then is compatible with the layer editor.

import ee
import geemap

m = geemap.Map()
col = ee.ImageCollection([ee.Image(1).byte(), ee.Image(10).byte()])
m.add_layer(col)
m

[github-actions]

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[jdbcode]

Thanks, Qiusheng!