Add new dump format map in aconfig

The `aconfig dump --format map` lists aconfig keys mapped to its boolean
values that represents whether the flag is enabled or not, as seen below:
```
flag.name1=true
flag.name2=false
```

Test: aconfig dump --format map --cache out/soong/.intermediates/build/make/tools/aconfig/aconfig.test.flags/intermediate.pb && inspect output
Bug: 306024510
Change-Id: Ic4990c168f6fa9c87869113ba695c07394adbc67