Install llndk.libraries.txt with llndk_in_system

llndk.libraries.txt was installed from generic_system.mk when VNDK is
deprecated. However, as variable KEEP_VNDK is not valid from
generic_system.mk, it did not work as expected. Anyway, now
llndk.libraries.txt is required for product VNDK deprecation, and also
it would be better to be added as a required module with LLNDK libraries
over llndk_in_system module. This change relocates defining
llndk.libraries.txt as required module into llndk_in_system module.

Bug: 290160925
Test: aosp_cf build succeeded with llndk.libraries.txt installed under
/system/etc

Change-Id: I482f94d701e5dc2b4f01308a65824b38ba5b9fde