Separate applicationId from package name

[brentvatne]

In https://github.com/expo/expo-cli/pull/2687 we introduced validation on the package name, which is correct and reasonable, however in the managed workflow package name actually more accurately means applicationId - it doesn't actually map to the package name used in the native project, and so different validation rules apply.

We should change this so that developers can specify the applicationId separately. For apps that already are using a package name that is invalid as an actual Java package name but valid as an applicationId, they can change the field name from package to applicationId and continue their development.

applicationId:

• It must have at least two segments (one or more dots).
• Each segment must start with a letter.
• All characters must be alphanumeric or an underscore [a-zA-Z0-9_].
• Maybe can allow Java reserved words? TBD

packageName:

• Same as above but...
• No Java reserved words
• Should only use lowercase characters