Add a few permissions required by fastbootd

These address the following denials:

avc: denied { getattr } for pid=547 comm="fastbootd"
path="/dev/block" dev="tmpfs" ino=18532 scontext=u:r:fastbootd:s0
tcontext=u:object_r:block_device:s0 tclass=dir permissive=1␍

avc: denied { read } for pid=547 comm="fastbootd" name="fstab"
dev="sysfs" ino=25820 scontext=u:r:fastbootd:s0
tcontext=u:object_r:sysfs_dt_firmware_android:s0 tclass=dir permissive=1␍␊

avc: denied { open } for pid=547 comm="fastbootd"
path="/sys/firmware/devicetree/base/firmware/android/fstab" dev="sysfs"]
ino=25820 scontext=u:r:fastbootd:s0
tcontext=u:object_r:sysfs_dt_firmware_android:s0 tclass=dir permissive=1

Bug: 119115481
Test: fastboot flashall
Change-Id: I97dc783ceef396145e3baa5d79194560fc0634f7