adb: extract atransport's connection interface.

As step one of refactoring atransport to separate out protocol handling
from its underlying connection, extract atransport's existing
hand-rolled connection vtable out to its own abstract interface.

This should not change behavior except in one case: emulators are
now treated as TCP devices for the purposes of `adb disconnect`.

Test: python test_device.py, with walleye over USB + TCP
Test: manually connecting and disconnecting devices/emulators
Change-Id: I877b8027e567cc6a7461749432b49f6cb2c2f0d7
diff --git a/adb/Android.mk b/adb/Android.mk
index 0eeafb6..e52f0cb 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -11,6 +11,7 @@
 adb_target_sanitize :=
 
 ADB_COMMON_CFLAGS := \
+    -frtti \
     -Wall -Wextra -Werror \
     -Wno-unused-parameter \
     -Wno-missing-field-initializers \