adb: Allow enabling of device side adbd logging with a persistent system property.

To enable logging, set the property persist.adb.trace_mask to a hex value
containing the bitmask for adb_trace_mask (see the TRACE_* enum values in adb.h).
This will result in adb writing log output to a file in /data/adb/
No logging will occur if persist.adb.trace_mask is not set or has a value
that cannot be parsed as a hex integer.
The property is read once only at startup, so you must reboot or restart adbd
for changes in the property to take effect.

Signed-off-by: Mike Lockwood <lockwood@android.com>