Always return /vendor/bin/sh as shell for OEM ids in getpwnam() etc.

During the review of the script that generates /vendor/etc/passwd, it
was suggested that the shell be /vendor/bin/sh instead of the typical
/system/bin/sh.  This has subsequently caused bionic unit tests to
fail, since they always check that the shell is set to /system/bin/sh.

In the spirit of that review, libc is modified to return
/vendor/bin/sh for the OEM AID ranges and the test is updated to
expect this.

Test: bionic unit tests
Change-Id: Ie7c1c48fde8a71b3df1aa0ef112d42ab7bd3baec