Make LocalTransport.DEBUG dynamic.

There's a general recommendation to make such static fields as FALSE
on framework code so they're removed by the compiler and don't take
space in memory, but LocalTransport is a "debug artifact", so that
memory increase is not a problem and the increase in APK size is
negligible, hence it's more flexible for developers to be able to
turned it on dynamically.

Test: adb shell setprop log.tag.LocalTransport VERBOSE && adb logcat LocalTransport:v *:s
Bug: 394173116
Flag: EXEMPT changes on debugging components only

Change-Id: I11d239f9b2a70de8f94ff7f9f6df0be79fc69d42