Atrace: Add commandline flags to enable/disable userland tracing

Adds a commandline flag to Atrace for Perfetto: --only_userspace.

When --only_userspace is passed atrace does everything atrace
would normally do to set up/tear down tracing (set various system
properties, poke services etc) but doesn't modify any ftrace
settings, doesn't clear read or otherwise touch the ftrace buffer,
and doesn't start or stop tracing.

Perfetto allows recording and collection of ftrace kernel events
(as systrace/atrace does for local devices). We currently integrate
with atrace to collect non-kernel events using the async_start and
async_stop command line arguments. These cause atrace to poke
binder/hal services and apps then Perfetto scrapes the print ftrace
events from the kernel ring buffer.

There are two downsides:
1. Atrace attempts drain the ftrace buffer after Perfetto has already
   done so.
2. Any time Atrace starts/stops it modifies the global ftrace settings and
   clears the ring buffer.

The second is especially unfortunate as otherwise Perfetto supports
multiplexing multiple simultaneous traces.

This flag would make it much easier to integrate atrace and Perfetto.

To test:

atest cts/hostsidetests/atrace

external/chromium-trace/systrace.py input sched

First enable ftrace:
$ adb shell 'echo 2048 > /d/tracing/buffer_size_kb'
$ adb shell 'echo > /d/tracing/trace'
$ adb shell 'echo 1 > /d/tracing/tracing_on'

Next turn on the input category.
$ adb shell atrace --async_start --only_userspace input
Touch the screen a bunch.
$ adb shell cat /d/tracing/trace
Now you should see many events.

Next turn off the input category.
$ adb shell atrace --async_stop --only_userspace
Clear tracing.
$ adb shell 'echo > /d/tracing/trace'
Touch the screen a bunch.
$ adb shell cat /d/tracing/trace
You should see no events.

Ftrace settings should not have been touched:
$ adb shell cat /d/tracing/tracing_on
Should be 1 still.
$ adb shell cat /d/tracing/buffer_size_kb
Should be 2048 still.

Bug: 73625407
Test: see above.
Change-Id: Ia43ab689c4f93280315eb984581187c1582c2b07
Merged-In: Ia43ab689c4f93280315eb984581187c1582c2b07