Disable marshal methods generation by default - #9336

Context: dotnet#8253
Context: dotnet#9343

We've had a number of reports about *hangs* when LLVM Marshal Methods
are enabled (see also dotnet#8253), but we had been willing
to accept having LLVM Marshal Methods enabled by default as it helps
improve app startup time.

However, [a customer reported a native crash][0] when LLVM Marshal
Methods are enabled:

> ![Thread Stack Trace][1]

	syscall at line 28 within libc
	__futex_wait_ex(void volatile*, bool, int, bool, timespec const*) at line 144 within libc
	sem_wait at line 108 within libc
	within libmonosgen-2.0.so (Build Id: …)
	within <anonymous:…>

This native crash suggests "something going wrong" within MonoVM.
We find this "scary" enough that we feel it is once again prudent
to disable LLVM Marshal Methods by default in .NET 9, by setting the
default value of `$(AndroidEnableMarshalMethods)`=False.

Even though dotnet#9343 has a potential fix for the hang, we feel that
we're too close to .NET 9 GA to validate in the wild.  This feature
will need to wait for .NET 10.  🙁

To explicitly enable LLVM Marshal Methods, set
`$(AndroidEnableMarshalMethods)`=True in your App `.csproj`.

[0]: https://discord.com/channels/732297728826277939/732297837953679412/1288758900627345463
[1]: https://cdn.discordapp.com/attachments/732297837953679412/1288758900522483712/05F6ED48-2BBA-4FC0-A54F-81BB57606500.png?ex=66f659c1&is=66f50841&hm=8f6f78f283f8c03e9fb37a452ad5d0babaaad3dd4bcc1b4887f4dcf60f651c12&

…9346)

Context: #8253
Context: #9343

We've had a number of reports about *hangs* when LLVM Marshal Methods
are enabled (see also #8253), but we had been willing
to accept having LLVM Marshal Methods enabled by default as it helps
improve app startup time.

However, [a customer reported a native crash][0] when LLVM Marshal
Methods are enabled:

> ![Thread Stack Trace][1]

	syscall at line 28 within libc
	__futex_wait_ex(void volatile*, bool, int, bool, timespec const*) at line 144 within libc
	sem_wait at line 108 within libc
	within libmonosgen-2.0.so (Build Id: …)
	within <anonymous:…>

This native crash suggests "something going wrong" within MonoVM.
We find this "scary" enough that we feel it is once again prudent
to disable LLVM Marshal Methods by default in .NET 9, by setting the
default value of `$(AndroidEnableMarshalMethods)`=False.

Even though #9343 has a potential fix for the hang, we feel that
we're too close to .NET 9 GA to validate in the wild.  This feature
will need to wait for .NET 10.  🙁

To explicitly enable LLVM Marshal Methods, set
`$(AndroidEnableMarshalMethods)`=True in your App `.csproj`.

[0]: https://discord.com/channels/732297728826277939/732297837953679412/1288758900627345463
[1]: https://cdn.discordapp.com/attachments/732297837953679412/1288758900522483712/05F6ED48-2BBA-4FC0-A54F-81BB57606500.png?ex=66f659c1&is=66f50841&hm=8f6f78f283f8c03e9fb37a452ad5d0babaaad3dd4bcc1b4887f4dcf60f651c12&

Co-authored-by: Marek Habersack <[email protected]>

It appears this section might have been AI generated, as it appeared
to contain "hallucinated" MSBuild property names. It also didn't make
sense that the example set things to `false`. I updated the section as
appropriate.

We also have made the feature non-default in:

* dotnet/android#9336

I also linked to GitHub commits, as we have more detailed text inside,
generally, than pull requests.

It appears this section might have been AI generated, as it appeared
to contain "hallucinated" MSBuild property names. It also didn't make
sense that the example set things to `false`. I updated the section as
appropriate.

We also have made the feature non-default in:

* dotnet/android#9336

I also linked to GitHub commits, as we have more detailed text inside,
generally, than pull requests.