[Xamarin.Android.Build.Tasks] use typemaps when EmbedAssembliesIntoApk=False

[jonathanpeppers]

This is a first step to use the typemap.mj and typemap.jm files
for Fast Deployment. We will eventually generate these files on a
per-assembly basis, but can get some small perf improvements for
Debug builds:

• When $(EmbedAssembliesIntoApk) is True use the native assembly
  in typemap.*.inc and the current behavior.
• When $(EmbedAssembliesIntoApk) is False don't generate any
  typemap.*.inc files at all, but still use the native assembly for
  environment variables.
• We have to still generate typemap.*.s files with empty values.

typemap.*.s changes

Before:

  jm_typemap_header:
    /* version */
    .long	1
    /* entry-count */
    .long	1324
    /* entry-length */
    .long	262
    /* value-offset */
    .long	117
    .size	jm_typemap_header, 16

    /* Mapping data */
    .type	jm_typemap, %object
    .global	jm_typemap
  jm_typemap:
    .size	jm_typemap, 346889
    .include	"typemap.jm.inc"

After:

  jm_typemap_header:
    /* version */
    .long	1
    /* entry-count */
    .long	0
    /* entry-length */
    .long	0
    /* value-offset */
    .long	0
    .size	jm_typemap_header, 16

    /* Mapping data */
    .type	jm_typemap, %object
    .global	jm_typemap
  jm_typemap:
    .size	jm_typemap, 0

The result is I don't get a crash at runtime and the typemap.mj and
typemap.jm files are used.

Results

I timed the Xamarin.Forms integration project in this repo.

An initial build:

Before:
2860 ms  GenerateJavaStubs                          1 calls
  84 ms  CompileNativeAssembly                      1 calls
After:
2595 ms  GenerateJavaStubs                          1 calls
  46 ms  CompileNativeAssembly                      1 calls

On an incremental build where MainActivity.cs was changed:

Before:
 543 ms  GenerateJavaStubs                          1 calls
After:
 323 ms  GenerateJavaStubs                          1 calls

CompileNativeAssembly is skipped for MainActivity.cs changes.

When comparing startup performance:

Before:
ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +1s302ms
After:
ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +1s341ms

As expected, startup is ~50ms worse.

I would say this change saves ~250ms on the full dev-loop in cases
where <GenerateJavaStubs/> needs to run.

[grendello]

@jonathanpeppers out of curiosity, what was the crash you got that this PR fixes?

[jonathanpeppers]

@grendello oh the crash was when I was trying to get this to work, sorry. It was before I made the right typemap.*.s changes.

[jonathanpeppers]

The only two test failures are zipalign on macOS and one related to lint on Windows. These should be unrelated, so merging.