Move special next handling of hidden APIs to java_sdk_library

Previously, the logic to add the Metalava command line option
`--hide-annotation android.annotation.FlaggedApi` when
`SOONG_SDK_SNAPSHOT_HIDE_FLAGGED_APIS=true` was done in
`droidstubs.go`. It worked but it turns out that we need to pass more
options to Metalava in that situation because while we need to hide
most flagged APIs we need to release some flagged APIs whose features
are ready. They cannot have the `@FlaggedApi` annotation removed
because that would cause the APIs to be released in the next QPR which
is not what we want.

So, in addition to the above option we will need to pass additional
options to Metalava to tell it to keep some flagged APIs. Rather than
hard code that in the build, requiring build changes for every new
SDK extension release this change uses the argument expansion mechanism
in Metalava (treating `@<file>` as if the contents of `<file>` were
passed as command line options) to delegate that to
":sdkext-released-flagged-apis".

The easiest way to do that was to move the logic to `sdk_library.go`
as `droidstub` would do the work of tracking the file dependencies
correctly. It also reduces the affect modules to `java_sdk_library`
modules rather than all `droidstubs` modules.

Bug: 313398274
Test: UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true vendor/google/build/mainline_modules_sdks.sh --build-release=next
Change-Id: Ib1cf22f8ef74abc135215537e9311936372eaac7
Merged-In: I4967376c0236bad729398af80fa59b48dbab5f21