[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…

[jonpryor]

Draft commit message:

Fixes: https://github.com/dotnet/java-interop/issues/1027

Context: https://github.com/dotnet/android/commit/8bc7a3e84f95e70fe12790ac31ecd97957771cb2
Context: 356485ee41fb61570e84c11ba120d6f8cf81ae09
Context: https://github.com/dotnet/android/pull/9724

`generator --codegen-target=XAJavaInterop1` -emitted marshal methods
can involve non-[blittable][0] types, specifically `bool` and `char`.
Consider:

	namespace Java.Nio.Charset;
	partial class CharsetEncoder {
	    public virtual unsafe bool CanEncode (char c) => …

	    static bool n_CanEncode_C (IntPtr jnienv, IntPtr native__this, char c) => …
	}

Historically this hadn't mattered, because we always had a runtime
marshaler.

This started changing with the introduction of LLVM Marshal Methods
in dotnet/android@8bc7a3e8: instead of the previous method registration
approach, in which a bunch of `Delegate` instances were collected and
passed to `JNIEnv::RegisterNatives()`, the idea was to instead emit
LLVM-IR for a `Java_…` native symbol which would lookup and invoke
a [`[UnmanagedCallersOnly]`][1]-attributed method, and
`[UnmanagedCallersOnly]` requires that all parameter and return types
be *blittable*.  There were two potential solutions:

 1. Add a wrapper around the marshal method which converts parameter
    and return types, or

 2. Skip them entirely, and require use of `JNIEnv::RegisterNatives()`
    for types overriding Java methods which involve `bool` or `char`.

dotnet/android@8bc7a3e8 went with (2).  ((1) is complicated.)

We now introduce solution (3): update `generator` so that the
generated marshal methods always use blittable types:

  * We marshal `bool` as `sbyte`
  * We marshal `char` as `ushort`.

`generator` has built in support for conversions when the native type
does not match the managed type, via `ISymbol.ToNative()` and
`ISymbol.FromNative()`.

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`;
see dotnet/android#9724.

[0]: https://learn.microsoft.com/en-us/dotnet/framework/interop/blittable-and-non-blittable-types
[1]: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.unmanagedcallersonlyattribute?view=net-6.0