adb: switch asocket::enqueue to std::string.

Switch asocket over to taking a std::string instead of apacket* for
data. This allows us to remove asocket specific fields from apacket*.

Test: python test_device.py with x86_64 emulator, walleye
Test: adb_test on host
Change-Id: I9d157ff331a75ba49a54fdd4194e3f6cdff722f4