[Configuration] Use :, not ,, as ABI separator.

[jonpryor]

Update $(AndroidSupportedAbis) and
$(AndroidSupportedAbisForConditionalChecks) to use : as the ABI
separator, not ,.

; can't be used because xbuild and MSBuild don't like ; as a
property value on the command-line.

Turns out, MSBuild doesn't like , within property values on the
command-line either, because it allows multiple MSBuild properties to
be specified via one /p: use:

$ msbuild -help
...
  /property:<n>=<v>  Set or override these project-level properties. <n> is
                     the property name, and <v> is the property value. Use a
                     semicolon or a comma to separate multiple properties, or
                     specify each property separately. (Short form: /p)
                     Example:
                       /property:WarningLevel=2;OutDir=bin\Debug\

This means that it's not possible to set $(AndroidSupportedAbis) to
e.g. host-Darwin,armeabi-v8a on the command-line with MSBuild.
(It is with xbuild, but this is arguably an xbuild compatibility bug!)

Since we want to be able to easily override the
$(AndroidSupportedAbis) value on the command-line for testing,
change the ABI separator character to : which is supported on both
xbuild and MSBuild:

$ xbuild /p:AndroidSupportedAbis=host-Darwin:armeabi-v7a
# works!

[radical]

Is this intentional?