SF: Add ImageHolder for screenshot code

Adds a simple ImageHolder class, which holds an EGLImage and destroys
it either when an explicit destroy() method is called or when the class
is destructed (whichever occurs first). This allows us to reduce the
nesting of, and otherwise simplify, the captureScreenImplLocked method
of SurfaceFlinger.

Bug: 62257775
Test: SurfaceFlinger_test and manually verified that screenshots still
      work
Change-Id: I800208e8f6bedd2ce66827268ecc64968dbe616d