Add support for dalvik.vm.extra-opts property.

The goal here is to avoid the dalvik.vm.* property explosion by
having a single property that takes an arbitrary collection of Dalvik
command-line options.  This is intended for testing of various
configurations by the Dalvik team, not industrial use.

Options should be separated by spaces, e.g.

  adb shell setprop dalvik.vm.extra-opts "-showversion -Xmx4m"

will print the version banner and set the heap max to 4MB, which won't
get you very far.

The extra-opts options will appear last, which allows them to override
values set earlier (like the heap max).

Bug 2838629.

(cherry-pick from dalvik-dev branch)

Change-Id: Ibcbb1b62367cf2152798583e8722ef7e461ad19a