[One .NET] Don't preserve all of Android.Runtime.JNIEnv (#5362)

Context: #5167

Some members of `Android.Runtime.JNIEnv` are accessed via Reflection,
and need to be preserved, but that doesn't mean the *entire type*
needs to be preserved.

Update the `<type/>` preservation XML so that only the required
methods are always preserved, allowing the linker to remove other
unused members.

src/Microsoft.Android.Sdk.ILLink/PreserveLists/Mono.Android.xml

@@ -7,7 +7,12 @@
 		<type fullname="Android.Runtime.IJavaObject" />
 		<type fullname="Android.Runtime.InputStreamAdapter" />
 		<type fullname="Android.Runtime.InputStreamInvoker" />
-		<type fullname="Android.Runtime.JNIEnv" />
+		<type fullname="Android.Runtime.JNIEnv">
+			<method name="Exit" />
+			<method name="Initialize" />
+			<method name="PropagateUncaughtException" />
+			<method name="RegisterJniNatives" />
+		</type>
 		<type fullname="Android.Runtime.JniNativeInterfaceStruct" />
 		<type fullname="Android.Runtime.JniNativeInterfaceInvoker" />
 		<type fullname="Android.Runtime.JNINativeWrapper" />