Add filename property to prebuilt_etc

filename property is an optional property that specifies the name of the
installed file which is by default name of the module.

This will be used to APEXs to have ./etc/ld.config.txt files for their
own.

Test: m (prebuilt_etc_test.go added)
Change-Id: Ic8d0c0044c5bc2c6c33117fe2c19ef6ad75451a8