Shrink Bundle.getString(key, default) and getCharSequence(key, default)

They previously duplicated the full logic of getString(key) and
getCharSequence(key) independently.  Now they both use the no-default
version internally,  resulting both in a single point of implementation
for test & isolation purposes and in a (small) decrease in code size.

Change-Id: If1b1e1fa4a549a0ec4cf221e54998f4fd438f3a2