Don't fall back to system driver when driver apk fails.

Previously when driver apk fails to load, the loader falls back to load system
driver. However, this provides no indication of driver apk failure and hence
users that intend to use driver apk may end up working against the system
driver.

BUG: b/147459984
Test: Verified by forcing to use a dummy apk
Change-Id: I3befc0d6c1f143dc53eab3762b7ff19c702e8cda