Remove letter tile's manual asset caching

In b/63143138 we discovered an issue with the LetterTileDrawable code.
The class has static fields that are shared across both the main and
background threads, which aren't threadsafe. We attempted to make the
code threadsafe in cl/160696979, but that ended up causing issues as
well.

This CL takes a different approach. Rather than trying to make the code
threadsafe, it instead removes the shared fields and makes them final
and initialized in the constructor. This is inherently threadsafe since
threads cannot interact with the object until it is constructed.

Now that the objects aren't shared, we would potentially use more
memory, since each LetterTileDrawable will now hold onto an instance of
the formally shared object. This was not determined to be an issue
in practice, see the data and conclusion in
(go/lettertiledrawable-caching).

Bug: 63143138
Test: LetterTileDrawableTest (can't verify everything :/ )
PiperOrigin-RevId: 161432711
Change-Id: I443bef01ffac405f5e14e9d981ce1d5a8f3f8fec