build: fix build/ and .gradle/ removal after cleaning
An app may not have a top-level build.gradle file, while Gradle still creates top-level .gradle/ directory. When build.gradle is absent, fdroidserver will not remove .gradle/, scanner will find binaries in it and fail the whole build.
Fix this by also checking for settings.gradle file (should always exist in the top-level directory) in addition to build.gradle.
Fixes #384 (closed).