Disable all host OS variants except those explicitly included.

Necessary to avoid problems with implicitly enabled host OS'es, e.g.
linux_glibc getting enabled when we only supply a linux_bionic linker
in runtime-module-host-exports. That will then cause a non-functional
prebuilt to take precedence over source if the prebuilt is preferred.

We don't do this for device since we so far only support a single
device OS (android).

This introduces the notion that SDK member types can be host OS
dependent or not. That way java members with host prebuilts don't get
restricted to a specific host OS.

Test: m nothing
Test: build/soong/scripts/build-aml-prebuilts.sh runtime-module-host-exports
  Check that the generated Android.bp correctly disables the bionic
  linker prebuilt for linux_glibc.
Test: art/build/apex/runtests.sh
  on master-art with an updated runtime SDK snapshot
Test: art/tools/buildbot-build.sh {--host,--target}
  on master-art with an updated runtime SDK snapshot
Bug: 160349757
Change-Id: Idad7ef138cdbcbd209d390bf6c10ca8365d4619f

android/sdk.go

@@ -327,6 +327,12 @@ type SdkMemberType interface {
 	// SdkAware and be added with an SdkMemberTypeDependencyTag tag.
 	HasTransitiveSdkMembers() bool
 
+	// Return true if prebuilt host artifacts may be specific to the host OS. Only
+	// applicable to modules where HostSupported() is true. If this is true,
+	// snapshots will list each host OS variant explicitly and disable all other
+	// host OS'es.
+	IsHostOsDependent() bool
+
 	// Add dependencies from the SDK module to all the module variants the member
 	// type contributes to the SDK. `names` is the list of module names given in
 	// the member type property (as returned by SdkPropertyName()) in the SDK
@@ -389,6 +395,7 @@ type SdkMemberTypeBase struct {
 	PropertyName         string
 	SupportsSdk          bool
 	TransitiveSdkMembers bool
+	HostOsDependent      bool
 }
 
 func (b *SdkMemberTypeBase) SdkPropertyName() string {
@@ -403,6 +410,10 @@ func (b *SdkMemberTypeBase) HasTransitiveSdkMembers() bool {
 	return b.TransitiveSdkMembers
 }
 
+func (b *SdkMemberTypeBase) IsHostOsDependent() bool {
+	return b.HostOsDependent
+}
+
 // Encapsulates the information about registered SdkMemberTypes.
 type SdkMemberTypesRegistry struct {
 	// The list of types sorted by property name.