[jnimarshalmethod-gen] Ask cecil to read assemblies to the memory

[radekdoulik]

That helps to process the assembly faster.

Process Mono.Android.dll times:

Before:

whole       37.34 real        37.29 user         2.04 sys
types        7.02 real         6.54 user         1.17 sys

After:

whole       33.84 real        34.99 user         1.01 sys
types        4.41 real         4.76 user         0.36 sys

Where whole means process all types in the assembly and types
means use the --types option to process only listed types.

The listed types:

# XA template startup, Mono.Android
#
Android.Runtime.UncaughtExceptionHandler
Java.Interop.TypeManager\+JavaTypeManager
Android.Views.View\+IOnClickListenerImplementor

# XForms template, Mono.Android
#
Android.Animation.ValueAnimator\+IAnimatorUpdateListenerImplementor
Java.Lang.Thread\+RunnableImplementor

[jonpryor]

Let's just make sure that we don't load jnimarshalmethod-gen.exe into an MSBuild process. :-)

Related: dotnet/android@fc6b7e9

...which does the opposite: moves from using InMemory=true to InMemory=false, which makes the <BuildApk/> task slower -- which is largely what you see in this PR -- but makes the overall build time faster, presumably because there are fewer/smaller GC's going on.

In the case of this PR, jnimarshalmethod-gen.exe exits relatively quickly, so presumably there doesn't need to be a GC or as many GC's.