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.