Run idmap in its own domain.

Run idmap in its own domain rather than leaving it in installd's domain.
This prevents misuse of installd's permissions by idmap.

zygote also needs to run idmap.  For now, just run it in zygote's
domain as it was previously since that is what is done for dex2oat
invocation by zygote.  zygote appears to run idmap with system uid
while installd runs it with app UIDs, so using different domains
seems appropriate.

Remove system_file execute_no_trans from both installd and zygote;
this should no longer be needed with explicit labels for dex2oat and
idmap.

Change-Id: If47e2c1326b84c20e94a20f5e699300dce12bdfe
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>