adb: don't spew to logcat when tracing is enabled.

When persist.adb.trace_mask is used to enable verbose logging, avoid
exponential logging when a user runs `adb logcat`. As a side-effect,
set the minimum logging level to include everything when ADB_TRACE is
used on the client (which is something we want anyway).

Bug: http://b/72971016
Test: `adb shell 'setprop persist.adb.trace_mask; killall adbd'; adb logcat`
Change-Id: Id4bca1f3933a920345499dbeaeb103c8a2e220cd