Detect out of memory in extractAlpha

When extractAlpha() native method in Skia fails to allocate pixels,
it resets target bitmap. This change detects when such empty bitmap
is returned, and throws a OutOfMemory Java exception.

Depends on https://android-git.corp.google.com/g/97793

Bug: 3418381
Change-Id: I65a84998be089c49ed5005f6995bdc4f4d1669bc