GIFMovie: Stop using internal Skia APIs

Bug: 178700363
Test: manual

Instead of using sk_memset32, which is private to Skia, use regular
memset. Skia recently moved sk_memset32, resulting in a build breakage.
But it's not obvious that this deprecated class needs the faster version
of memset.

Also remove an include of SkTemplates, which is no longer used.

Change-Id: Iae6b495e5aace3ea92d2fc59798ea3fd23d3b1b9