aconfig: package.map file serialization

For each container, there will be three storage files: package.map,
flag.map and flags.val. This change adds package.map creation.

package.map is a hash table in file that maps a package name to its
package id and package value offset. This file will be used by flag
storage client lib to locate the fixed offset of a flag in flag value
file. package.map provides the file byte offset to the start of all flag
values for this package. Together with flag value offset within its package
which will be provided by flag.map, we can locate a flag's value in the
value file.

The top level struct for this file is called "PackageTable". The struct
consists of three parts: (1) table header which includes the file
version and other metadata such as number of packages and etc. (2) table
buckets which is an array of u32, each bucket stores the package.map
file offset to the package table node. (3) package table node array.
each node stores package name, package id, package value offset and
offset to the next package table node.

The table uses fixed format serialization. All u32 are encoded using
little endian format as that is the Android format. All strings are UTF8
encoded. This is to ensure cross platform compatibility.

Bug: b/312243587
Test: atest aconfig.test
Change-Id: I1041405db42862573ec320c0e557948732c28eb8