Context: dotnet/java-interop#719

Modify "old" bindings, so that they hold the object reference
long enough to avoid above issue.

It is implemented as linker step, which looks for offended methods
and injects call to System.GC.KeepAlive to prevent premature
collection.

The injection can be disabled with AndroidAddKeepAlives property
set to false.

The property's default value depends on AndroidIncludeDebugSymbols
property and is thus true for release builds and false for debug.

The impact on release build of XA forms template (flyout variety) is
cca 137ms on my machine.

Disabled:

12079 ms  LinkAssemblies                             1 calls

Enabled:

12216 ms  LinkAssemblies                             1 calls

Example updated method body:

try {
	JniArgumentValue* ptr = stackalloc JniArgumentValue[1];
	*ptr = new JniArgumentValue((windowToken == null) ? IntPtr.Zero : ((Java.Lang.Object)windowToken).Handle);
	ptr[1] = new JniArgumentValue((int)flags);
	bool result = JniPeerMembers.InstanceMethods.InvokeAbstractBooleanMethod("...", this, ptr);
} finally {
	GC.KeepAlive(windowToken);
	return result;
}

Added new AndroidAddKeepAlives test to LinkerTests.cs to check
whether registered method was updated.