Revert to version 2.51 of DroidSansFallback.ttf

Version 2.53 of DroidSansFallback.ttf included:
    1) Currency: \u060B\u09F2\u09F3\u09FB\u0AF1\u0BF9\u17DB\u20B6-\u20B9
       \uA838\uFDFC
    2) Geometric right arrow: \u25BA
    3) CJK Ext A (about 6000 Simplified Chinese Glyphs)

Item 3 (the ~6000 Simplified Chinese Glyphs) increased the file size by
about 1.6M.  To reduce ROM pressure, we are reverting back to version
2.51.  Version 2.53 has been renamed to DroidSansFallbackFull.ttf and is
available for builds that have enough ROM space.  We are generating a
new version of DroidSansFallback.ttf that has items 1 and 2 and not 3 and
will update DroidSansFallback.ttf once we have that version.

Bug: 5239728
Change-Id: I7467109a9ffec452446a636d35a3befffff09f10