Allow system services to be optional

Some Android systems may not have certain system services (particularly the VirtualMachineManager). If this is the case, disable the preference rather than requiring the service and crashing the Settings app.

Bug: 388221800
Test: manual
Flag: NONE bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:5a594828d7e148ac98b077591e8050e424b54966)
Merged-In: I761054caebe3e846f5c6c76e2818662aad2cb511
Change-Id: I761054caebe3e846f5c6c76e2818662aad2cb511