Use stem when filtering boot jars.

For testing purposes, a boot jar may be provided by a test java_library
that has a different content name, but sets "stem" property to match
the original java_library.

Given that Stem() returns either the property value or module name,
it is safe to replace all content names by their stems.

Bug: 180105615
Test: atest CtsClasspathsTestCases sdkextensions_e2e_tests
Change-Id: Ic519ffa0c5b616abddf15b41c934421dfac2e78a

java/base.go

@@ -1778,3 +1778,9 @@ type ProvidesUsesLib interface {
 func (j *Module) ProvidesUsesLib() *string {
 	return j.usesLibraryProperties.Provides_uses_lib
 }
+
+type ModuleWithStem interface {
+	Stem() string
+}
+
+var _ ModuleWithStem = (*Module)(nil)