Restrict maximum size of FontInterpolator font caches

This fixes a memory leak where Fonts were leaked within detailed lock
screen animations. We're occasionally animating fonts via several axes
which causes a low occurence of cache hits. This resulted in hundreds of
Font instances being kept in the cache.

Bug: 275486055
Test: Verified behaviour on cheetah
      Expanded and ran unit tests for affected classes
      Benchmarked rendering time with reduced cache - increase of
      duration of draw() command for 0.3ms with 10 item cache.
      50 item cache increased draw() command duration for 0.15ms on
average which is not worth the tradeoff in memory.

Change-Id: Id6e2b127f44f5e553ff7ac4ba640fd4f6f4698b0