Convert to Android.bp; rename libminijail_vendor

libminijail_vendor is now just libminijail, now that it's been converted
to Soong.

See build/soong/README.md for more information about Soong.

Test: mmma hardware/interfaces/minijail
Change-Id: Iefa49fbbd96c1ae18c5da4367e2c9135d8c25a5c