Remove assumption that _PATH_BSHELL is a compile-time constant.

The alternative would be to build sshd using the (V)NDK, and I initially
went down that route but the because (a) we build sshd for API level 19
and (b) there are a couple of dependencies on non-NDK libraries/headers,
that's a significantly larger and more complicated change.

One day, when we don't have to support anything older than API 23 we
can remove most/all of the Android-specific changes in this project,
at which time we should drop this change and just build with the (V)NDK.

Bug: http://b/67975799
Test: builds
Change-Id: If6be7947e69b24cfa514a17bee6fef3cfbddc236