llkd: add ro.llk.sysrq_t

Allow sysrq stack trace dump to be disabled by ro.llk.sysrq_t.

Default is true if not on a limited memory device ro.config.low_ram.
Value is true if the property value is "eng" and on a userdebug or
eng device, signaled by the ro.debuggable set to 1.

Test: compile
Bug: 118712403
Change-Id: I02e999dc640125b6a08dca10077716e5d006da49