Increase the maximum shell command length to 4096ish.

The actual maximum length will depend on the version of the shell
protocol being used, and any additional parameters being passed through
(e.g. TERM=xterm-256color). This should be able to be raised to 64K for
devices with commit 3d2904cd (L-MR1 and above), but that'll require some
plumbing.

Bug: http://b/20467103
Change-Id: Idf0c46af5b18b854110aba58df13a53297d2475f