Allow uninstallable prebuilt_etc to be packaged

Till now, prebuilt_etc has to be installable to be packaged. With this
commit, packaging modules like filesystem now can include uninstallable
prebuilt_etc. This can help when prebuilt_etc can conflict with other
partitions e.g. system.

Bug: 179340780
Test: build filesystem with uninstallable prebuilt_etc
Change-Id: Id981fe7ce41f1189eb0f85f2d1677d973004423c