ANDROID: build: cuttlefish: Fix path to clang.

Reconcile with changes made to the kernel manifest. Clang must come from
master because it was not usable for kernel builds in older branches of
the Android platform.

Bug: 63889157
Change-Id: Id0a080fc2f1cba495f37f26afa48e43e736b756a
Signed-off-by: Alistair Strachan <astrachan@google.com>