Generate runtime stubs in droidstubs

Similar to how the exportable stubs are generated in droidstubs, this
change adds the rule to generate the runtime stubs in droidstubs. Unlike
the exportable stubs, which the artifacts are dist'ed during the sdk
build, the runtime stubs will only used in the hiddenapi flags
generation. Therefore, only the stubs are generated and no other
artifacts are generated during the "runtime" metalava invocation. When
`aconfig_declarations` property is not defined (i.e. no flags are
enabled, thus the exportable and the runtime stubs are identical), the
runtime stubs are simple copied from the exportable stubs.

Note that the runtime stubs are not a dependency of `droid` as of now.
The dependency of the hiddenapi flags will be replaced to the runtime
stubs in the child CL.

Test: m nothing --no-skip-soong-tests && m api-stubs-docs-non-updatable
Bug: 319162970
Change-Id: I14928b7b5619522c4caab1dfc795bc9c7e929639