Context: #789

One issue with the proposal to flip the order of `ApiXmlAdjuster` and
`metadata` in Issue #789 is that the "apply metadata" step is
intertwined with the "parse API xml document into POCO's" step.
This means that the adjuster step cannot be inserted between them,
as the code is currently written.

Separate out the two steps so that they can be reordered in the future.

Additionally, refactor the metadata fixup step so that it can be unit
tested more easily by moving it to `Java.Interop.Tools.Generator.dll`.
Unit tests for applying metadata have been added.

Context: dotnet/android#5638
Context: dotnet/android#5879

Going forward in .NET 6, we shouldn't use the `%(IsTrimmable)`
metadata anymore, but instead use the following C# attribute in
each assembly for which linking/"trimming" is supported:

	[assembly: AssemblyMetadata ("IsTrimmable", "True")]

Similar changes on the iOS side here:

  * dotnet/macios@289053b

Add `AssemblyMetadataAttribute` to `Java.Interop.dll`.