Add aconfig flag value text file in aconfig_declarations provider

Alongside with the generated proto file, the aconfig_declaration now
also outputs a text file that lists aconfig flags and values of its
corresponding proto file, in the format as shown below:
```
my.flag1=true
my.flag2=false
...
```

To prevent confusion between the preexisting proto file and the newly
introduced text file, the change also renames the variables of the proto
file from `intermediatePath` to `intermediateCacheOutputPath` and
likewise.

The utilization of the generated text file will be done in the child
changes.

Test: m out/soong/.intermediates/build/make/tools/aconfig/aconfig.test.flags/intermediate.txt && inspect output
Bug: 306024510
Change-Id: Iee16ad57bb87e992a477fc96502f79e971d01233