Pass Bitmap's native instance to JNI where feasible

Test: CtsGraphicsTestCases, CtsUiRenderingTestCases,
      CtsRenderscriptTestCases

This is significantly faster than passing the Java object down and then
calling a JNI method to retrieve the pointer. See
https://buganizer.corp.google.com/issues/16656908#comment19

In some cases this changes what used to be native crashes (due to
android::BitmapWrapper:assertValid's LOG_ALWAYS_FATAL_IF) into
NullPointerExceptions (if a caller used a null Bitmap).

In addition:
- Remove unnecessary JNIEnv param from toBitmap(jlong)
- Change instances of toBitmap(JNIEnv*, jobject) to the above
- Replace calls to GraphicsJNI::getSkBitmap() to inline calls to
  toBitmap/getSkBitmap
- make Canvas#nInitRaster @FastNative (FIXME: Could these be
  @CriticalNative?)

Change-Id: I6194097be1b6e6952eba70e1e7052a5a250eed93