perfetto: gate system-backend registration in the Java SDK

PerfettoTrace.register(false) consults TracingPolicy before connecting to the
system tracing backend. If the process is not permitted it returns false and
leaves Perfetto uninitialized, so no producer and no muxer thread are created for
it. register() returns whether tracing is usable; callers skip category
registration and event emission when it is false. No extra state is added:
emission already short-circuits on Category.isEnabled(), so categories that are
never registered are no-ops.

The SDK is compiled once and jarjar'd into three variants, so TracingPolicy is a
single class that selects behaviour from its own (jarjar'd) package name, the
same way PerfettoNativeLibrary does:
- libcore (dalvik.system.): allows only system_server and com.android.systemui.
- framework (com.android.internal.): allows a larger allowlist, a comma-separated
  sysprop allowlist, or an "enable all" override.
- default (host / tests): unrestricted.

System properties are read reflectively so the SDK keeps no dependency on
android.os.SystemProperties (absent in the default/libcore compile and
off-device). The process name is read from /proc/self/cmdline: the framework
caches the same value as Process.myProcessName (both set by Process.setArgV0),
but that isn't reachable here, and Files.readAllBytes can't be used because /proc
files report a size of 0.

Change-Id: I354807c2ed14cfd6f94aa39cc83be6963db57bca