Move adb backup/restore helptext to adb shell bu help.

Key/value support for adb backup was added in ag/1946677, and help text
for adb shell bu is added in ag/2088937. This CL points the user to this
helptext instead of keeping it here, since it can quickly get out of
sync.

BUG=36170696

Test: adb
Change-Id: Ic40679f421a32e60f292ad20b726a89bfff03349