fix update-makefiles.sh to not need Android.mk

When auto-generating Android.mk for HIDL packages, update-makefiles.sh
now just looks for all directories hosting .hal files, converts them to
HIDL package names, and invokes hidl-gen on these package names to
generate the appropriate Android.mk files.

Change-Id: I8a824672f33be58508ae7c07de463f4f589a9a73
Signed-off-by: Iliyan Malchev <malchev@google.com>