[Java.Interop.Tools.TypeNameMappings] remove Assembly.GetName().Name usage

[jonathanpeppers]

Context: https://github.com/jonathanpeppers/Benchmarks/blob/cc3f2fb1c8f34c30b5b89b2a2b7e327ab7743bb1/Benchmarks/FullyQualifiedName.cs

I found Assembly.GetName().Name is somewhat slow compared to using
Assembly.FullName and a Substring.

It was an order of magnitude faster on .NET framework on Windows:

Intel Core i9-9900K CPU 3.60GHz, 1 CPU, 16 logical and 8 physical cores
  [Host]     : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.8.4075.0
          Method |      Mean |     Error |    StdDev | Gen 0/1k Op | Allocated Memory/Op |
---------------- |----------:|----------:|----------:|------------:|--------------------:|
       Substring |  92.05 ns | 0.1823 ns | 0.1616 ns |      0.0168 |                88 B |
 AssemblyGetName | 949.68 ns | 1.9551 ns | 1.8288 ns |      0.0896 |               473 B |

But still quite faster on Mono on macOS:

Intel Core i7-6567U CPU 3.30GHz (Skylake), 1 CPU, 4 logical and 2 physical cores
  [Host]     : Mono 6.6.0.155 (2019-08/296a9afdb24 Thu), 64bit
          Method |     Mean |     Error |    StdDev | Gen 0/1k Op |
---------------- |---------:|----------:|----------:|------------:|
       Substring | 1.596 us | 0.0040 us | 0.0035 us |      0.0687 |
 AssemblyGetName | 2.182 us | 0.0110 us | 0.0097 us |      0.2289 |

*NOTE: macOS is displaying us (microseconds) and Windows ns (nanoseconds)

I suspect the reason is that AssemblyName splits the string parsing
the Version, PublicKeyToken, etc. We don't need those values in
some cases.

JavaNativeTypeManager.GetPackageName is heavily used during the
build in <GenerateJavaStubs/>:

• When generating Java stubs
• When generating AndroidManifest.xml
• For the acwmap.txt and other typemaps

It is also used at runtime by the Android designer.

After making the change, I could see a small improvement when building
the Xamarin.Forms integration project:

  Before:
  386 ms  GenerateJavaStubs                          1 calls
  After:
  361 ms  GenerateJavaStubs                          1 calls

This seems like it will save ~25ms on builds when
<GenerateJavaStubs/> runs.

I have seen <GenerateJavaStubs/> take over a second with some larger
projects, so it is probably worth making this change.