Fix pattern rules for $(installed_odex) for libdvm.

When the VM is libdvm, we don't put the odex files in an arch specific
subdirectory. The previous pattern rules don't work because of the extra
"/". With this change, % evaluates to empty string when it's built for
libdvm; % evaluates to "<arch_name>/" when it's built for libart.
Also removed use of $(create-empty-package), which may causes file name
(dummy) conflict with the rule of package.apk.

Bug: 15311527
Change-Id: I9f9089bc1896b78c1f47834afdb28a3a51d34480