Introduce getSourceIdentifier instead of isSameSource.

After some experiment, equality function is not enough for
high performance animation. To use native glyph cache efficiently,
need to cache the Font object during font variation animations.
This identifier will be used for the key of the cache.

Bug: 171911683
Test: atest FontTest
Change-Id: Idbaa1bc83aea2c6af82641bd7d3a553f2cfe4793