AAPT2: Clean up R JavaDoc generation

- Don't generate private attributes in public R.java
- Strip out @SystemApi from comment when generating @android.annotation.SystemApi
- Only emit a single line (up to the first period) of an attribute's comment within
  a styleable's attribute table.

Change-Id: Id6316a6861540325934133958939a12074ad4428
8 files changed