Soong: libbpf_prog: change output filetype to .bpf

Changing libbpf_prog output types to .bpf to differentiate from legacy
bpf.o files. Also, removing libbpf from the output directory.

Bug: 359646531
Test: manual/TH
Change-Id: I9ee22099acf57920bc5054ecf42e209a7b6418b6
Signed-off-by: Neill Kapron <nkapron@google.com>