Convert the rest of libbacktrace to Android.bp

See build/soong/README.md for more information.

Reapplies I61c49fb62fdeafb66db25b8f97621239b8fd6e75 after fixing
an unused parameter warning on mips builds.

Test: mma -j
Test: backtrace_test
Change-Id: I6dc353a274cafcdb05ee866b41c3c2731612941c