Manually escape BuildNumberFromFile

Change ctx.Config().BuildNumberFromFile() to be unescaped, and then
escape it manually where necessary.  This will allow passing
ctx.Config().BuildNumberFromFile() to RuleBuilder, which will handle
the necessary escaping.

Test: no change to out/soong/build.ninja
Change-Id: I5329ab2ba70ccea49958864488aa81794a7e4b56