Add min_sdk_version property to apexes

Add a min_sdk_version property apexes.  Currently a noop, but will
be used to enforce that dependencies are compatible with the
specified version.

Test: m checkbuild
Bug: 149591522
Change-Id: I923773c90fe15becbffae3986791aa9edde8f8f6

apex/apex.go

@@ -1280,6 +1280,9 @@ type apexBundleProperties struct {
 	// rules for making sure that the APEX is truely updatable. This will also disable the size optimizations
 	// like symlinking to the system libs. Default is false.
 	Updatable *bool
+
+	// The minimum SDK version that this apex must be compatibile with.
+	Min_sdk_version *string
 }
 
 type apexTargetBundleProperties struct {