Tighten up Binder.clearCallingIdentity() usage.

The recently added AndroidFrameworkBinderIdentity Error Prone checker
examines code to ensure that any cleared identities are restored to
avoid obscure security vulnerabilities.

This change is a purely mechanical refactoring that adds the "final"
keyword to the cleared identity to ensure that it's not accidentally
modified before eventually being cleared.  Here's the exact command
used to generate this CL:

$ find . -name "*.java" -exec sed -Ei \
    's/    (long \w+ = .+?clearCallingIdentity)/    final \1/' \
    {} \;

Bug: 155703208
Test: make
Exempt-From-Owner-Approval: trivial refactoring
Change-Id: I832c9d70c3dfcd8d669cf71939d97837becc973a