Add flagged api hide conditional to droidstubs

APIs annotated with @FlaggedApi should not be included in the artifact
when building sdk target products in the "next" release configuration.

This change adds such logic by passing additional flag to metalava in
droidstubs.

The flag does not need to be passed to metalava invocation done in
java_api_library, as java_api_library generates stubs using api
signature files (i.e. *-current.txt files), and they will not contain
apis marked @FlaggedApi. The metalava invocation in droidstubs is
responsible for removing such apis.

Test: go test ./java && m TARGET_PRODUCT=sdk TESTING_TARGET_RELEASE_NEXT=true nothing and inspect ninja command for generating stubs and verify the flag is included
Bug: 299570421
Change-Id: Ia4b699b6e3ff6324f050eecc9ff5b622fdc04621