core/jni/android/graphics: Cache copy of pre-scaled Bitmaps

Caching some pre-scaled Bitmap objects allows for optimizing later
scaling operations to reduce memory pressure.

CRs-fixed: 451681

Change-Id: Id4341e75911074ff9f565b2d386ccad8f3a223e8
Function uri.getAuthority is called twice. Minor doc corrections.

Function uri.getAuthority was called twice in methods acquireProvider
and acquireExistingProvider was called twice although a parameter
representing the value had existed. The second call to the function is
changed to the parameter. The parameter's modifier changed to final.
Minor corrections in function descriptions in the file.

Signed-off-by: Yury Zhauniarovich <y.zhalnerovich@gmail.com>
Change-Id: Id003aa38c17d644357873c41a8f5ec455e46a4b7