Fix ext4 image's root directory permissions of APEX packages.

The generateFsConfig rule used to not give execution
permissions (0111) to the root (`/`) directory of the ext4 image
stored in an APEX package. This change add these bits, turning the
permissions from 0644 (drw-r--r--) to 0755 (drwxr-xr-x).

Previously, not having the execution permissions on the ext4 image's
root directory meant that the contents of the top-level directory of
the mounted image could not be listed.

Test: `adb shell ls /apex/<apex-package>` works
Bug: 117580281
Bug: 112672359
Bug: 113373927
Change-Id: I96f859de6022e04c0b82ccdf1db683887643e588