whisper.android : return output from benchmarks (#1785)

whisper.android : return output from benchmarks (#1785)

Benchmarks are failing because JNI expects a jstring and the benchmarks
are missing a return statement (i.e., returning null). The functions
actually build a jstring but don't return it, so this seems to have been
an oversight.

This patch returns the jstring and now the benchmarks run successfully.

Fixes #1783.