Cache .so files for debugging per device and don't redownload .so's that are in your cache

BUG=
R=eseidel@chromium.org, eseidel

Review URL: https://codereview.chromium.org/1158493005

tools/android_library_cacher.py

@@ -57,6 +57,8 @@ def main():
         dest_dir = os.path.dirname(dest_file)
         if not os.path.exists(dest_dir):
             os.makedirs(dest_dir)
+        if os.path.exists(dest_file):
+            continue
         print '%s -> %s' % (library_path, dest_file)
         pull_cmd = [ADB_PATH, 'pull', library_path, dest_file]
         subprocess.check_call(pull_cmd, stderr=dev_null)

tools/skydb

@@ -37,7 +37,8 @@ ANDROID_APK_NAME = 'MojoShell.apk'
 
 PID_FILE_PATH = "/tmp/skydb.pids"
 CACHE_LINKS_PATH = '/tmp/mojo_cache_links'
-SYSTEM_LIBS_ROOT_PATH = '/tmp/device_libs'
+# TODO(iansf): Fix undefined behavior when you have more than one device attached.
+SYSTEM_LIBS_ROOT_PATH = '/tmp/device_libs/%s' % (subprocess.check_output(['adb', 'get-serialno']).strip())
 
 SRC_ROOT = skypy.paths.Paths('ignored').src_root
 ADB_PATH = os.path.join(SRC_ROOT,