adb: increase the shell command length limit.

Relax the shell command length limits when talking to an adbd with the
shell protocol.

shell is pretty much the only service that takes an arbitrarily long
string, so this is somewhat safe.

Bug: http://b/37716055
Test: `adb shell $(python -c 'print "echo " + "f" * (32*1024)') | wc` on L and master

Merged-In: I0737fd2244530ef8080f300cd3a3549a1ab93465
Change-Id: I0737fd2244530ef8080f300cd3a3549a1ab93465