Fix "supermarket" missing from PlaceTypes

[breyed]

Fixes #401. This is a breaking change: the type of Google.Places.Types is changed from PlaceType[] to string[].

Advantages:

• Works around a bug in Google's SDK, which includes supermarket in the supported types, but does not include it in GMSPlaceType.
• Adds resilience against future change to the Google SDK. If a type is added that is not known to the Xamarin wrapper, the wrapper's code will get an unexpected string, which is much easier to deal with than the getter for Places.Types throwing NotSupportedException.

Disadvantages:

• When developers update to the new NuGet package, they will get a compile error and have to change their code. This is preferable, however, to their code compiling and the system appearing to work, only to have apps mysteriously crash in the field as users search happen to search for supermarkets.

If the PR is implemented, PlaceType should be removed, as it is no longer uses, and the documentation should be updated to link to the supported types.

[breyed]

Report for bug in Google SDK: No constant for "supermarket" place type

[breyed]

For consistency, if this PR is implemented, In Google.Places.AddressComponent, the property Types should be changed to type string[]. The property Type should be removed: it is already obsolete; a breaking-change transition is a good time to remove it.

[SotoiGhost]

@breyed Thanks for your contribution and thanks for linking to the right docs!

Breaking changes are bad for everyone. Let me check the code to see if there's a way to avoid the pain.