Move listeners and callbacks inside GoogleMap

That’s where they’re located in Google Play Services.
Fix typo
Implement getCenter() in LatLngBounds
Fix applicationId
Remove appcompat dependency
Move WebView logic to an inner view of MapView
Fix colors not respecting transparency

It seems the default opacity is around 0.5. Setting it to 1.0 and then
using rgba() seems like a more solid approach.
onMapLoaded() should only be triggered once
Add README note about Google Maps API key
Convert stroke width to pixels

The specified dimension is already supposed to be in pixels, but due
to drawing in a WebView, it gets converted from dp to px. Do the inverse
which cancels it out.