adb: implement LZ4 compression.

Add support for LZ4 compression, which compresses and decompresses far
more quickly than brotli, at the cost of worse compression ratio.

`adb sync -d system` speeds (in MB/s) on aosp_blueline-eng:

           none    brotli    lz4
USB 3.0     120       110    190
USB 2.0      38        75     63

Bug: https://issuetracker.google.com/150827486
Test: python3 -m unittest test_device.FileOperationsTest{Uncompressed,Brotli,LZ4}
Change-Id: Ibef6ac15a76b4e5dcd02d7fb9433cbb1c02b8382
diff --git a/adb/test_device.py b/adb/test_device.py
index 496a0ff..3be7c9a 100755
--- a/adb/test_device.py
+++ b/adb/test_device.py
@@ -1302,6 +1302,10 @@
     compression = "brotli"
 
 
+class FileOperationsTestLZ4(FileOperationsTest.Base):
+    compression = "lz4"
+
+
 class DeviceOfflineTest(DeviceTest):
     def _get_device_state(self, serialno):
         output = subprocess.check_output(self.device.adb_cmd + ['devices'])