aconfig: update storage file to store flag index instead of file byte offset

Previously, in package.map, we store the byte offset of the first flag
in a flag package in a flag value file. In flag.map, we store the within
package flag byte offset. Once we know the total offset, we can locate
the flag in the flag value file. However, this offset may not work for
flag info file. Currently we only have boolean flags. Each boolean flag
value and info takes up 1 byte in flag value and flag info file. So the
byte offsets are the same. But in the future, when we add a new flag
value type, or add more contents to flag info file. Then the byte offset
for flag value file and flag info file may not be same.

Instead, we should store index rather than offset. The package map
should store the index of the first flag in this package. The flag map
stores the within package index. Then flag value read api and flag info
read api can then infer the byte offset independently.

Bug: b/321077378
Test: atest -c
Change-Id: Iad89776d5369715ba2d8faaea18261ac3271f13b