[Xamarin.Android.Build.Tasks] !Embed Assemblies == Debug Runtime

[jonpryor]

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=56485

Scenario: Create an app, then change $(Optimize) in the Debug
configuration to be True, with $(EmbedAssembliesIntoApk)=False
(default for Debug configuration).

Build, deploy, run the app.

Expected results: it works!

Actual results: Not so much:

F/monodroid: No assemblies found in '(null)' or '<unavailable>'.  Assuming this is part of Fast Deployment. Exiting..

What went wrong is Commit 1e0e083: making $(Optimize) the
switch to control whether the Debug or Release runtime is used means
that a "Debug" configuration with $(Optimize)=True is treated as
requiring a Release runtime, which means the external assembly
directory isn't used and doesn't exist.

Attempt to address this by bringing $(EmbedAssembliesIntoApk) into
the picture: if it's False -- i.e. fast deployment -- then it
"overrides" the $(Optimize) value when it comes to the
$(AndroidIncludeDebugSymbols) property.

(The $(AndroidIncludeDebugSymbols) property controls whether the
Debug or Release runtime is used. Yes, this name doesn't make sense.)

This means for greater sanity we now need a truth table:

Input Property:                                                      || Output Property
DebugSymbols    | DebugType  | EmbedAssembliesIntoApk   | Optimize   || AndroidIncludeDebugSymbols
================+============+==========================+============++===========================
True              *any*        True                       True          False      (Release runtime)
True              *any*        True                       False         True       (Debug runtime)
True              *any*        False                      True          True       (Debug runtime)
True              *any*        False                      False         True       (Debug runtime)
True              *empty*      True                       True          True       (Debug runtime)
True              *empty*      True                       False         True       (Debug runtime)
True              *empty*      False                      True          True       (Debug runtime)
True              *empty*      False                      False         True       (Debug runtime)
False             -            -                          -             False      (Release runtime)

Question: Should $(DebugSymbols) truly be the controlling factor,
i.e. when $(DebugSymbols)=False the Release runtime is always used.

Additionally, update $(AndroidUseDebugRuntime) so that it also
takaes $(EmbedAssembliesIntoApk) into consideration.