Fix android.Expand and ninja escaping

RuleBuilder does its own ninja escaping, so values that will be
passed to RuleBuilder must not be pre-escaped.  Add a new
android.ExpandNinjaEscaped method that explicitly handles ninja
escaping.  Some of the expansion functions return ninja values
(like "${in}") that need to stay unescaped, so add a bool return
value to the expansion function in android.ExpandNinjaEscaped.

Test: expand_test.go
Change-Id: Ib03d4db38b5e3e5bffbd87acf14f55e276a53d04