Expose the SkipInstall flag.

Necessary in e.g. install hooks to filter out module variants that
aren't installed.

Test: m nothing
Bug: 176192921
Change-Id: I67f6d7b4197f9d0c13239af1aa8ab439c21d7d6c

android/module.go

@@ -454,6 +454,7 @@ type Module interface {
 	InstallForceOS() (*OsType, *ArchType)
 	HideFromMake()
 	IsHideFromMake() bool
+	IsSkipInstall() bool
 	MakeUninstallable()
 	ReplacedByPrebuilt()
 	IsReplacedByPrebuilt() bool
@@ -1398,6 +1399,12 @@ func (m *ModuleBase) SkipInstall() {
 	m.commonProperties.SkipInstall = true
 }
 
+// IsSkipInstall returns true if this variant is marked to not create install
+// rules when ctx.Install* are called.
+func (m *ModuleBase) IsSkipInstall() bool {
+	return m.commonProperties.SkipInstall
+}
+
 // Similar to HideFromMake, but if the AndroidMk entry would set
 // LOCAL_UNINSTALLABLE_MODULE then this variant may still output that entry
 // rather than leaving it out altogether. That happens in cases where it would