Ignore failures in cleaning android projects

If the android test projects hasn't been built yet, the "make clean"
command will stop at "ant clean" (which fails since build.xml doesn't
exist yet). If this happens, one still would want to run the "make
clean" command to the end, cleaning up any possible intermediate object
files (e.g. from a build for a different platform).
Merge pull request #1068 from mstorsjo/android-clean

Ignore failures in cleaning android projects