Allow tracing service to access kallsyms on userdebug

This CL allows the traced_probes service to temporarily
lower kptr_restrict and read /proc/kallsyms.
This is allowed only on userdebug/eng builds.
The lowering of kptr_restrict is done via an init
property because the kernel checks that the kptr_restrict
writer is CAP_SYS_ADMIN, regardless of the /proc file ACLs [1].

[1] https://github.com/torvalds/linux/blob/4cbffc461ec91287c4cb1d0e27b01b988d0b8fba/kernel/sysctl.c#L2254

Bug: 136133013
Design doc: go/perfetto-kallsyms
Test: perfetto_integrationtests --gtest_filter=PerfettoTest.KernelAddressSymbolization in r.android.com/1454882

Change-Id: Ic06e7a9a74c0f3e42fa63f7f41decc385c9fea2c
diff --git a/private/property_contexts b/private/property_contexts
index 361db3e..ac680a4 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -44,6 +44,7 @@
 log.tag                 u:object_r:log_tag_prop:s0
 log.tag.WifiHAL         u:object_r:wifi_log_prop:s0
 security.perf_harden    u:object_r:shell_prop:s0
+security.lower_kptr_restrict u:object_r:lower_kptr_restrict_prop:s0
 service.adb.root        u:object_r:shell_prop:s0
 service.adb.tls.port    u:object_r:adbd_prop:s0
 persist.adb.wifi.       u:object_r:adbd_prop:s0