Weak symbol AIBinder_{setRequesting,getCalling}Sid

Add __attribute__((weak)) on header of the symbols. Otherwise, the
caller of APIs will not be able to compile successfully.

Bug: 159135973
Bug: 162055050
Test: atest libbinder_ndk_unit_test
Change-Id: I8d6c346874aa58620521c51ad5cf7d8c54fa24fb