Add cache validation to ensure the validity

Create cache identity from GL_VERSION and store in ShaderCache.
In the next time ShaderCache is restored from disk, compare the
cache identity to ensure its validity. If GL_VERSION changes in
between, flush out entire FileBlobCache and start from an empty
one.

Bug: b/71800782
Test: Wrote a new unit test to save and restore ShaderCache
Test: hwui_unit_tests
Change-Id: Ie573dc4f18733eee090725be30445d879765231b