Restore the ability to track native Surface changes

Bug #8230990

ViewRootImpl needs to know when the native Surface objects changes
to recreate the EGL surface. A recent refactoring in Surface broke
the behavior of getGenerationId(). This simply restores the old
behavior (every change increments the generation ID by 1.)

Change-Id: Ife1df1ffb2ee7a373b8ebf2431192702ba10f344