Set the atrace clock to boot when possible and mono otherwise.

Bug: 32379831
Test: Inspected the trace_clock while running atrace, without including the .rc file change:
$ adb shell
marlin:/ # cat /d/tracing/trace_clock
[local] global counter uptime perf mono boot
marlin:/ # atrace --async_start freq
capturing trace...
marlin:/ # cat /d/tracing/trace_clock
local global counter uptime perf mono [boot]
marlin:/ # atrace --async_stop > /dev/null
marlin:/ # cat /d/tracing/trace_clock
local global counter uptime perf mono [boot]
marlin:/ # exit

Test: Inspected the trace_clock while running atrace, after the .rc file change:
$ adb shell
marlin:/ # cat /d/tracing/trace_clock
local global counter uptime perf mono [boot]
marlin:/ # atrace --async_start freq
capturing trace...
marlin:/ # cat /d/tracing/trace_clock
local global counter uptime perf mono [boot]

Change-Id: I9ec88df109b311b90c9d88fe3a70f9ce090b4d15