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