Fix run-as which was broken in Android 4.3

In Android 4.3 the run-as binary no longer has the SUID/SGID bits
set. Instead, it requires to be installed with setuid and setgid
file-based capabilities. As a result of the above two changes, the
binary no longer executes as root when invoked by the "shell" user
but can still change its UID/GID to that of the target package.

Unfortunately, run-as attempts to chdir into the target package's
data directory before changing its effective UID/GID. As a result,
when run-as is invoked by the "shell" user, the chdir operation
fails.

The fix is for run-as to chdir after changing the effective UID/GID
to those of the target package.

Bug: 10154652
Change-Id: I48ecfeab7cd36991968d49e2382ceb4110694709