Make an Android-specific copy of Skia's Null GL interface

This functionality has been removed from Skia, but is still being used
by HWUI's NullGlesDriver for unit tests. Making a local copy gets tests
passing (so that rolling can resume).

Test: hwui_unit_tests still pass
Change-Id: I16b65af58eaeca5a39df89d1542760b652d8b0d5