fastbootd: Add getvar max-fetch-size.

Test: run it
Test: see follow up CL on fuzzy_fastboot

Bug: 173654501

Change-Id: I5ed110c5569d83cbe791d04b4abea3a2af2765a9
diff --git a/fastboot/device/commands.h b/fastboot/device/commands.h
index c1324bc..108ad2f 100644
--- a/fastboot/device/commands.h
+++ b/fastboot/device/commands.h
@@ -20,6 +20,7 @@
 #include <vector>
 
 constexpr unsigned int kMaxDownloadSizeDefault = 0x10000000;
+constexpr unsigned int kMaxFetchSizeDefault = 0x10000000;
 
 class FastbootDevice;