Generate "exportable" stubs in droidstubs

This change generates rules for "exportable" stubs in the droidstubs
module.

Given that there are a lot of overlap between the currently existing
"everything" stubs rule and the newly introducing "exportable" stubs
rule, the currently existing metalava rule commands are modularized to
be utilized in the "exportable" stubs rule commands.

The currently existing build actions are modularized in the followings:
- commonMetalavaStubCmd(...): metalava commands that are required for
  generation of both "everything", "exportable", and potentially
  "runtime" stubs
- everythingOptionalCmd(...): metalava commands that are dependent on
  "everything" stubs and not dependent on flagged apis annotations, such
  as api lint, released api check

Based on this modularization, the "everything" stubs are now generated
in everythingStubCmd(...), which calls commonMetalavaStubCmd(...) and
everythingOptionalCmd(...).
Similarly, the "exportable" stubs are generated in
optionalStubCmd(stubsType=Exportable, ...), which calls
commonMetalavaStubCmd(...) and appends additional flags. Runtime stubs
can be generated similarly in the future with
optionalStubCmd(stubsType=Runtime, ...).

"everything"-related artifacts will now  be created in
`everything/` subdirectory, and "exportable"-related artifacts will be
created in `exportable/` subdirectory. For example, the outdir of a
droidstubs module "foo" would look like below:
```
foo
  |-- everything
  | |-- foo_api.txt
  | |-- foo-stubs.srcjar
  |
  |-- exportable
    |-- foo_api.txt
    |-- foo-stubs.srcjar
```

The module generates the build rules for the "exportable" stubs
regardless of whether the module defines the `aconfig_declarations`
property or not. All APIs marked with `@FlaggedApis` annotations are
stripped out for the "exportable" stubs when the `aconfig_declarations`
property is not defined. On the other hand, only the `@FlaggedApis` that
are specified in the aconfig_declarations module and are enabled will be
included (and all others are stripped) when the `aconfig_declarations`
propety is defined.

Test: go test ./java && BUILD_FROM_SOURCE_STUBS=true m
Bug: 315490657
Change-Id: I300273cd2a62fa978b046c0268e3a67c35e22b08