[NativeAOT] set environment variables before the app .so is loaded

[grendello]

Fixes: #10705

When a NativeAOT application starts up, certain portions of the managed startup
sequence are executed on the shared library load but before the JNI_OnLoad
function is invoked by Android.

Both MonoVM and CoreCLR hosts set environment variables which may affect the
way the application behaves before they initialize the managed runtime. Those
variables are set from within native code, using data generated at application
build time.

Until now, the NativeAOT host followed suit, but it appears that we need to
set certain variables (e.g. those related to running the startup hooks) before
the shared library is loaded.

The only location where we can do it is the NativeAndroidRuntimeProvider Java
class which is responsible for, eventually, loading the application shared
library.

This commit makes our build process emit application environment variables into
a Java file that's compiled together with the NativeAndroidRuntimeProvider
class, at the same time removing those variables from the native code so that
we don't set the variables twice (a performance optimization, nothing more).

Testing shows that startup hooks are properly called with only the DOTNET_STARTUP_HOOKS
variable set, which wasn't previously the case.

[jonathanpeppers]

is there some existing native code that needs to be disabled for NativeAOT? that is packing and setting env vars at startup?