API dumps: fix preamble

Before the '{}' in the preamble command wasn't getting substituted, so
we were seeing this error:

[ 87% 7/8] //system/tools/aidl/asdf:an_aidl_1-api Freezing AIDL API of an_aidl_1 as version 1
cat: {}: No such file or directory

Now, we are using xargs so that when the command fails, the build will
fail (easier to find a problem in the future), and the path gets
substituted.

Bug: N/A
Test: freezing an API adds the preamble there (and not to the .hash
  file)

Change-Id: Ifbfacf9d21c77ccb88623be8839bf5ae1f3b3c46
1 file changed