ANDROID: GKI: Do not modify protected exports source list

Header generation script is using the protected exports list
as a source to generate the header file during the kernel build.
Script preporcess the symbols in-place before using it to generate
an array of symbols for protected exports causing the source file
to change. This may force the kleaf to build kernel again even
though there are no real changes in terms of symbols.

Use a copy as a temp file for processing leaving the source file
un affected.

Unprotected symbol list is already a temp file; so it doesn't
affect that target.

Bug: 268678245
Test: TH
Change-Id: Ifb551639451d1c7bd935ff732bd1959647c014d7
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>