Expose injectAndGetClassName and defaultSelectorHandlers

When we started using rollup to build this package (Aphrodite v2, https://github.com/Khan/aphrodite/pull/281), things that were previously deep-importable are no longer accessible by consumers. In 2.1.0 we exposed `flushToStyleTag` to un-break react-with-styles-interface-aphrodite (https://github.com/Khan/aphrodite/pull/310) but while working to update some things I also noticed that react-with-styles-interface-amp-aphrodite has some other deep imports. Specifically `injectAndGetClassName` and `defaultSelectorHandlers`. Although I'd like to keep the API surface area as minimal as possible, I think we should probably expose these so folks can get on the new version.

https://github.com/airbnb/react-with-styles-interface-amp-aphrodite/blob/e57d403f542c58861dcadb5a29e0834daf70d4d7/src/ampAphroditeInterfaceFactory.js#L1-L2

When we do this, let's also remember to expose them in no-important. See https://github.com/Khan/aphrodite/issues/311

cc @majapw

Start by inspecting the Rollup package entry points and the exports used for the standard and no-important builds. Expose injectAndGetClassName and defaultSelectorHandlers in both builds, then check the package output or existing export tests to confirm consumers can access them.

Written by the indexing model from the issue text.