Add Binder.clearCallingIdentity to TracingServiceProxy

The TracingServiceProxy is called by traced, which runs as UID 9999 and
therefore doesn't have the required permissions to start a foreground
service.

So, clear that calling identity so that the identity checked for this
permission is system_server, which does have the correct permissions.

We'll ensure that no other processes can utilize this path via selinux
rules.

Bug: 191391382
Test: Manually tested that before this change, I saw an
'ActivityManager: startForegroundService() not allowed' error when
taking a bugreport while a trace is running, while after this change the
bugreport was taken successfully with no errors, and the trace was
included in the bugreport.

Change-Id: I472fe8acc2e59e93afd8475f51b5f347cd3ccc5d