[generator] Avoid non-blittable types in native callback methods

[jpobst]

Fixes: #1027

To make marshaling callbacks from Java easier we should avoid non-blittable types in native callback methods. This means we should marshal bool as sbyte and char as ushort.

generator has built in support for conversions when the native type does not match the managed type. (ISymbol.[To|From]Native). However this affects marshaling both ways: managed to Java and Java to managed. We do not want to affect managed to Java marshaling, so we introduce a new ISymbol.OnlyFormatOnMarshal property to control this.

This also requires some changes in dotnet\android via this PR: dotnet/android#9724.

[jonpryor]

Looks like char will need to be replaced with uint16 as well; see e.g. https://github.com/dotnet/runtime/blob/3ab8574307f12f4afbb813fbe708734334f87914/docs/design/libraries/LibraryImportGenerator/Compatibility.md#char-marshalling

[char marshaling] will not be supported when configured with any of the following:

• …
• No explicit marshalling information…