Remove "StringTest.java".

This only tests stuff that's already better tested in dalvik/libcore, plus
private ICU API (RuleBasedNumberFormat) we'd like to remove, and that this
is the only caller of.

Conflicts:

	tests/CoreTests/android/core/StringTest.java

git cherry-pick -e 935022a99dac5454fc478e221b6392f3ba467009

Change-Id: I0bd172f2703bbcd6381920d40244165234fef4a1