Enable exportable stubs to include READ_WRITE aconfig flagged apis

Currently in SDK build, "exportable" stubs are used to generate the
android.jar and the corresponding build artifacts, as well as the
hiddenapi flags. "exportable" stubs only include the flagged apis that
are "enabled" and "read only", and exclude all other flagged apis. This
will be replaced with "runtime" stubs in the long run, which include
"read_write" flagged apis on top of the "enabled" and "read only" flags.

Prior to Trunk Stable, the SDK build did not distinguish the target
audience of the SDK. That is, the identical build target was used to
generate the SDK targeted toward the Google3 developers (i.e. Google3
SDK drop) and the SDK targeted toward the public (i.e. Developer
Preview). However, given that we now have "experimental" apis with Trunk
Stable, there are demands to differentiate the SDK based on the target
audience, so that the "experimental" APIs are included in the SDK
targeted toward Google3 while they are excluded in the public facing
SDK.

The long term solution to achieve this is to generate the hiddenapi
flags and (conditionally) the SDKs using the runtime stubs. However, as
this is high priority, this change resolves the problem by modifying the
filter condition of the "exportable" stubs to include the "read_write"
flagged apis on top of the "enabled" and "read only" flagged apis when
the value of the default-false build flag "RELEASE_EXPORT_RUNTIME_APIS"
is set to true.

Note that this is a temporary solution; However, we might need to keep
the "RELEASE_EXPORT_RUNTIME_APIS" build flag even in the long run to
determine what set of apis are included in the generated SDK, based on
the target audience of the SDK.

Test: m nothing --no-skip-soong-tests
Bug: 323188988
Change-Id: If0d5fa74b3ba6f4a57c86aade8d340f149a657a2