Prebuilt_etc can be uninstallable

For APEXs, we need different prebuilt_etc modules having the same
'filename' properties, because we expect them to have same local path
inside APEXs. For example, we will have ./etc/ld.config.txt file for
APEXs having an executable.

However, this can cause duplicated targets in the make world, because
the prebuilt_etc modules will all be installed to the same path under
/system.

In order to avoid this, adding 'installable' property to prebuilt_etc
module type to optionally make a module to be non-installable, but only
to APEXs.

Test: build/soong/build_test.bash --dist

Change-Id: Iadb564e07d0483934548ca63f5f524a2c8515a81