[Xamarin.Android.Build.Tasks] Fix $(EmbedAssembliesIntoApk)

[dellis1972]

Commit 73dc058 removed the default for $(EmbedAssembliesIntoApk) if
it is blank. This was not the right thing to do :).

This commit adds the a default of 'True' is $(EmbedAssembliesIntoApk)
if completely emplty by the time it has gone through all the other
processes.

[jonpryor]

The macOS+xbuild PR Build failure is due to our favorite error condition:

Error executing task BuildApk: Non-negative number required.
Parameter name: srcOffset
Error executing task BuildApk: System.ArgumentOutOfRangeException: Non-negative number required.
Parameter name: srcOffset
  at System.Buffer.BlockCopy (System.Array src, System.Int32 srcOffset, System.Array dst, System.Int32 dstOffset, System.Int32 count) [0x0009d] in /Users/builder/data/lanes/5263/mono-mac-sdk/external/bockbuild/builds
ass/corlib/ReferenceSources/Buffer.cs:65 
  at System.IO.FileStream.WriteSegment (System.Byte[] src, System.Int32 src_offset, System.Int32 count) [0x00023] in /Users/builder/data/lanes/5263/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/corlib/Sys
m.cs:995 
  at System.IO.FileStream.WriteInternal (System.Byte[] src, System.Int32 offset, System.Int32 count) [0x00099] in /Users/builder/data/lanes/5263/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/corlib/System
s:647 
  at System.IO.FileStream.Write (System.Byte[] array, System.Int32 offset, System.Int32 count) [0x00090] in /Users/builder/data/lanes/5263/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/Fi

  at System.IO.StreamWriter.Flush (System.Boolean flushStream, System.Boolean flushEncoder) [0x0007e] in /Users/builder/data/lanes/5263/mono-ma
ockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/io/streamwriter.cs:318 
  at System.IO.StreamWriter.Write (System.Char[] buffer, System.Int32 index, System.Int32 count) [0x00078] in /Users/builder/data/lanes/5263/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/io/streamwriter.cs:411 
  at System.IO.TextWriter.WriteLine (System.String value) [0x00070] in /Users/builder/data/lanes/5263/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/io/textwriter.cs:490 
...

I cannot wait until we replace xbuild use with msbuild everywhere...

Merging.