Don't add API annotations in the internal R.java

I'm trying to enable a check for the following structure:
```
/** @hide */
public class Class1 {
    /** @hide */
    @SystemApi // Invalid because the class is hidden.
    public void method1() { }
}
```

The internal R.java file violates this, which this change is going to fix.

Bug: 159162473
Bug: 163419414
Test: build (treehugger)
Test: atest aapt2_tests

Exempt-From-Owner-Approval: Cherry-pick from goog/master
Merged-In: I613e8611ddaf5f8e4761d351d4cd0142d59c7cc9
(cherry picked from commit de6e6f20)
Change-Id: Ia63f4e1026bc7f3df2e6bc7bbec02ee88a925e54