Allow obfuscated classes on bootclasspath

Previously, the signature_trie python library would reject classes
which started with a lower case character which in turn caused the
verify_overlaps tool to fail. That meant that it was impossible to
add obfuscated classes (which commonly used lower case characters
for their name) from being used on the bootclasspath.

This change removes that restriction and the accompanying test.

Bug: 265833521
Test: TH and partner testing
Change-Id: I70710484e427f64d79fb30301f3413f3b67b27e7