Fix bootimg.cmdline to use ShellEscapeIncludingSpaces

bootimg.cmdline should be passed to mkbootimg's --cmdline flag as a
single argument. Because cmdline can have spaces we should escape it
with ShellEscapeIncludingSpaces rather than blindly putting double
qoutes with ShellEscape, which also puts single quotes when necessary.

Bug: 182092664
Test: m nothing && see build.ninja (microdroid's build commands)
Test: boot microdroid on VIM3L
Change-Id: I5d766bb0a816d829569c9ebfddf97c4f846f78f8