Clean up NinePatch(Drawable) for mainline module

Bug: 155661807
Test: make

Document/remove @hide/@UnsupportedAppUsage from NinePatch(Drawable)
- Remove the unused copy constructor
- Remove @UnsupportedAppUsage from InsetStruct. No usage was shown on
  the dashboard; it was likely added by mistake due to its being called
  from JNI
- Document that the other @hide methods are needed for access from
  outside the package, but still within the module.

Change-Id: Ia27a5eda63a0115da54225f7ac18b3d2a55ab01f