Android backend: allow targets to have unmangled names.
The Android backend mangles gyp target names because the Android build
system requires that they be globally unique. This is fine for most targets,
but for the final build products that are being used by other Android
components it's annoying to refer to long mangled names. Add a
generator-specific flag to denote that a target should have an unmangled
name, which can be set on the targets that are important after verifying
that no existing module in Android has the same name.