Disable ptrace debugging by default on userdebug

I2fb1aa0aea81400f573b8c2e2fa309b7942d9446 re-enabled ptrace on all
apps by default on userdebug after I6fff6f1a683893244bc2d40f930044747befb2ce
had disabled it along with JDWP.  The same reasoning that applied to
JDWP also applies to ptrace, it could be trivially used for root
detection.  Return to defaulting it to off again, with an additional
persist.debug.ptrace.enabled property to turn it back on.

Bug: 292818059
Bug: 291332760
Bug: 290832806
Test: CtsMemunreachableTestCases
Change-Id: I0b8c07da661b7bf45c32b304aaa72cf9837dabd5