Rework class loader context implementation.

The old representation consisted of a list of libraries (UsesLibraries),
a list of optional libraries (OptionalUsesLibraries) and a mapping from
library name to its build/install paths (LibraryPaths). The separation
into lists and map was necessary because of special handling of
compatibility libraries, which is now unified with normal libraries.

The new representation is a mapping from target SDK version to a tree
structure ClassLoaderContext. Each node of the tree represents a library
and contains library name, build/install paths and a slice of
subcontexts for dependencies. The same library may occur in the tree
multiple times in case it is a dependency of multiple libraries. The
order in which libraries are added matters (the resulting tree shape may
be different).

Test results have to be updated, as the resulting <uses-library> list is
reodered (previously it was a sorted list of map keys, and now it is
formed by a depth-first preorder traversal of the class loader tree).

Test: lunch aosp_cf_x86_phone-userdebug && m
Bug: 132357300
Bug: 168686456
Change-Id: I11be8cd2967f004fd58753d7c5fb99fed179cd63

java/java_test.go

@@ -1593,8 +1593,8 @@ func TestJavaSdkLibrary(t *testing.T) {
 	// test if baz has exported SDK lib names foo and bar to qux
 	qux := ctx.ModuleForTests("qux", "android_common")
 	if quxLib, ok := qux.Module().(*Library); ok {
-		sdkLibs := android.SortedStringKeys(quxLib.ExportedSdkLibs())
-		if w := []string{"bar", "foo", "fred", "quuz"}; !reflect.DeepEqual(w, sdkLibs) {
+		sdkLibs := quxLib.ExportedSdkLibs().UsesLibs()
+		if w := []string{"foo", "bar", "fred", "quuz"}; !reflect.DeepEqual(w, sdkLibs) {
 			t.Errorf("qux should export %q but exports %q", w, sdkLibs)
 		}
 	}