fs_mgr: Look for fstab file in /system/etc

Look for the fstab file in /system/etc as an alternative to /, in order
to allow fstab files to be installed using the "prebuilt_etc" Soong
module.

This new path is meant to be used by the vendor ramdisk only. As before,
fstabs should *not* be placed in /system/etc on the system partition.

In more detail: sometimes, multiple nearly-identical fstabs need to be
installed to a device, with the correct one being selected at boot time
(b/191417025 as well as other cases that partners have run into).  To
avoid error-prone duplication of configuration files, these fstabs
should be generated from a template by the build system instead of being
duplicated in the source tree.  But if this is done, the usual way of
installing fstabs (PRODUCT_COPY_FILES) can't be used; they need to be
made into real build system modules instead.

Currently, the "prebuilt_etc" Soong module can't correctly install the
vendor_ramdisk copy of the fstab(s), since it will install it into the
/system/etc directory whereas Android currently requires that the
vendor_ramdisk copy of the fstab(s) be placed in the root directory.

Earlier I proposed adding a "prebuilt_fstab" module to handle this quirk
(https://r.android.com/1744033).  However, it was requested to instead
always look for the fstabs in /etc too, in order to allow "prebuilt_etc"
to be used and because /etc is the appropriate place for this file.
This change implements that suggestion (but actually using /system/etc,
since that is where "prebuilt_etc" actually installs it).

Bug: 191417025
Test: Tested that a device boots both with this, both before and after
      http://ag/15075136 which uses the new location.
Change-Id: Id083070e51ae85959167e4615cd96b31a0b1bd6a