Limit the size of a ScreenNail.

bug:6528366
This bug was happened because we are trying to make a texture beyond the
max size allowed in GL.

Here is what we do in this CL:
1. Limit the size of a screen nail
2. Print warning message, if we try to allocate a texture beyond the size
3. Don't show fall-back animation if the image is not loaded yet.

Change-Id: I004b1138efd0eef7ba11aa89556f67743ca46745