screencap: fix a memory leak

If fork fails, we'll leak this strdup. The strdup here seems
unnecessary; just use a pointer to the string itself instead.

Caught by the static analyzer:
frameworks/base/cmds/screencap/screencap.cpp:119:8: warning: Potential
leak of memory pointed to by 'fp_arg' [clang-analyzer-unix.Malloc]

Bug: None
Test: TreeHugger
Change-Id: Id7c738ed5c090bcb5b13a6258e91f027ff2b778c