cmdline property in bootimg can be customized for debuggable builds

This change adds product_variables.debuggable.cmdline so that the kernel
cmdline arguments can be augmented for the debuggable builds.

To support that the type of the property has changed to string array.

Bug: 181936135
Test: inspect build.ninja
Change-Id: I81b3ead5187ccb378efabb87d1d15fbdb59d8e2f