Clean up easy Bitmap dependencies

Bug: 150395371
Test: make

- Mark mDensity as package-protected, since it is not used elsewhere.
- Document getNativeInstance, getOpticalInsets, getNinePatchInsets,
  and scaleFromDensity - where they are called, and that they should not
  be called from outside the module.
- Mark setImmutable as private. No outside callers use it.

Change-Id: I85efe4829e2914222696aa659f8a21b699c3d788