Remove exports_manifest attribute of android_library

[ahumesky]

The exports_manifest attribute of android_library is leftover from an internal migration. It should be removed from android_library and manifests should always be merged up the build graph.

[ahumesky]

Ah, this is already half done: https://github.com/bazelbuild/bazel/blob/a5376aa3e11ea2ecfbd52068794ed3e652d9c179/src/main/java/com/google/devtools/build/lib/rules/android/AndroidLibraryBaseRule.java#L106

[ahumesky]

Ah, but this was not carried over to the starlark implementation: https://github.com/bazelbuild/rules_android/blob/7c8e4ac72668ad869072236ea615450347a5dad4/rules/android_library/attrs.bzl#L89