Add test case for setSystemFallback API

We cannot expect anything about the behavior of setSystemFallback since
everything is hidden internally.
The only things we can expect is
- NPE if null is passed
- Any string is fine in terms not crashing.

Bug: 120866864
Test: atest TypefaceCustomFallbackBuilderTest
Change-Id: I355ffb9ef1405042d268ffccbf204a7323d4e9af
1 file changed