sign_target_files_apks: Fix .prop string replacment

Value was being assigned unconditionally and causing a type mismatch (list and str)
Instead use a temporary value variable and only assign to value if the condition is
true.

Change-Id: I1b7d9dfd89321b23c51acf9fb19ceb37b530749e
Signed-off-by: Evan Anderson <evan1124@gmail.com>