build.gradle: remove usage of GRADLE_VLC_SRC_DIRS / add vlcLibVariant property

It was set by vlc-android, based on VLC_OUT_PATH.

VLC_OUT_PATH is set to /build-android-/ndk and only contains temporary files:

• libvlcjni-modules.c
• libvlcjni-symbols.c

They are not used in the java build.

There is no VLC_OUT_PATH/libs folder after libvlcjni is built. libvlcjni.so is in /jni/libs which we already handled.

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The GRADLE_ABI environment variable is only set by the vlc-android build script. But it could also be passed as a project property to avoid relying on dependencies on the environment.