aconfig: update aconfig_storage_read_api

Previously we are ensuring that the public rust api to get mapped file
is safe as we are ensuring that the file being mapped has a permission
of 444. However, a file permission of 444 is not possible due to build
system needs to make file 644 while creating img files. Thus need to make the rust api to get mapped file unsafe.

In reality, this should have no impact. Because, even though the storage
files have a file permission of 644, they are on a RO partition like
system. So the files are not writable anyway. This is true for all the
containers (platform partitions and mainline modules) we know so far.

Bug: b/321077378
Test: atest aconfig_storage_read_api.test; atest
aconfig_storage_read_api.test.rust; atest
aconfig_storage_read_api.test.cpp

Change-Id: I643fe191e697a524e2303a32750f04c268f408fd