Refactor merged annotations creation

It's clearer to define the module in Android.bp, and keep
our custom Go code to the minimal. The only thing the go
code really needs to do is create a "all module annotations"
module. This matches the pattern of the other modules here (the
pattern was invented after sdk-annotations.zip was first
migrated).

Bug: 235140934
Test: m sdk-annotations.zip before/after (no diffs)
Change-Id: Iae246081bcf4e5177d1da98c2e01c702fb349a22