Remove extern "C" from header files

Since change 290046, most files in system/bt is compiled as C++ source.
Therefore, there is no longer a need for the extern "C" wrapper around
includes that export symbols from those sources.

The following python script is applied to each file in the directory:

front = '#ifdef\s+__cplusplus\s+extern\s+"C"\s+{\s+#endif\s+'
back = '#ifdef\s+__cplusplus\s+}\s+#endif'
with open(sys.argv[1], "r") as f:
  data = f.read()
  data = re.sub(front, "", data)
  data = re.sub(back, "", data)
  print data

through a shell script:

for file in $(find . -name "*.h"); do
  python remove_cpp_extern_c.py $file > tmp
  cat tmp > $file
  rm tmp
done

with following files not edited:
* stack/include/a2dp_*
* include/bt_trace.h
* embdrv/sbc/*

Bug: 33492510
Test: Code compilation, BtFunhausMetricsTest, BtStressTest
Change-Id: Iac21cdfb1924b50478dd0b82326e092602cbc9d4