dashboard: add known issues for android-*-emu

Issue golang/go#42212 manifests as test timeouts, and is by far the most
frequent of these known issues.

Issue golang/go#51001 causes failures with "systemstack called from unexpected
goroutine". It seems to have been introduced sometime last year, but
it isn't clear to me whether it is a regression or an older (latent)
bug unearthed by some other change.

Issue golang/go#52724 appears to be a bug or race in the Android emulator
itself. It might require a builder image update and/or escalation to
the maintainers of the emulator proper.

Updates golang/go#53377.

Change-Id: I677915b1ff02dd02e0f14c63b0d25caf11e27a72
Reviewed-on: https://go-review.googlesource.com/c/build/+/412174
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

dashboard/builders.go

@@ -2463,6 +2463,7 @@ func init() {
 			"GOHOSTOS=linux",
 			"CGO_ENABLED=1",
 		},
+		KnownIssues: []int{42212, 51001, 52724},
 	})
 	addBuilder(BuildConfig{
 		Name:              "android-amd64-emu",
@@ -2493,6 +2494,7 @@ func init() {
 			"GOHOSTOS=linux",
 			"CGO_ENABLED=1",
 		},
+		KnownIssues: []int{42212, 51001, 52724},
 	})
 	addBuilder(BuildConfig{
 		Name:     "illumos-amd64",