Add droidstubs support for merging show/hide annotations.

This works like the existing annotation merging support, but uses a
different flag, because the merging needs to happen earlier in
metalava (see the other change in this topic).

Bug: 115746226
Test: `make api-stubs-docs`
Test: `make core-platform-api-stubs` with local modifications to trigger this path
Change-Id: I30d6d7993e06b105fb3e9fcefde39c1bf3da998a