aconfig: create memory map file module

This module is repsonsible for finding the storage file location based on a
pb file and then map these storage files into memory using memmap2
crate. The mapped files are cached in a static hashmap in this module.

Note that memmap2 crate has not been introduced to AOSP yet as a 3p
crate. It is in the process of importing to AOSP ATM. So in src/lib.rs,
we only include this module if we are building the target using cargo.

Also, the test storage files: package.map, flag.map and flag.val are
generated based on the storage test aconfig files in
aconfig/aconfig/tests dir. With aconfig create-cache command first to
produce cache files, and then aconfig create-storage command to create
each storage file.

aconfig create-cache --package com.android.aconfig.storage.test_1 --container system --declarations ./storage_test_1.aconfig --cache com.android.aconfig.storage.test_1.cache

aconfig create-cache --package com.android.aconfig.storage.test_2 --container system --declarations ./storage_test_2.aconfig --cache com.android.aconfig.storage.test_2.cache

aconfig create-cache --package com.android.aconfig.storage.test_4 --container system --declarations ./storage_test_4.aconfig --cache com.android.aconfig.storage.test_4.cache

aconfig create-storage --container system  --cache ./com.android.aconfig.storage.test_1.cache --cache ./com.android.aconfig.storage.test_2.cache --cache ./com.android.aconfig.storage.test_4.cache --file flag_val --out ./flag.value

aconfig create-storage --container system  --cache ./com.android.aconfig.storage.test_1.cache --cache ./com.android.aconfig.storage.test_2.cache --cache ./com.android.aconfig.storage.test_4.cache --file package_map --out ./package.map

aconfig create-storage --container system  --cache ./com.android.aconfig.storage.test_1.cache --cache ./com.android.aconfig.storage.test_2.cache --cache ./com.android.aconfig.storage.test_4.cache --file flag_map --out ./flag.map

Bug: 321077378
Test: atest aconfig_storage_file.test
Change-Id: Id4bd26ce1a2513cdda201b479cec2b8c04faabfe