Fix for not ending up in an eternal loop in DrmManager.

In the original code a random number is used to get try
to find an empty slot in the list of free DRM id's.
When you reached the limit of 0xfff id's you ended up
in an eternal loop causing ANRs.

Updated by James Dong <jdong@google.com>.

Change-Id: I70176cc3f770223c4a8060f9739fe2bc03a703d9