Update platform namespace name

Platform namespace has been renamed as 'system' from linkerconfig
generator. To meet this requirement, libnativeloader should search for
namespace 'system' rather than namespace 'platform'.

Bug: 147987608
Test: m -j passed
Test: atest libnativeloader_test passed
Change-Id: I23d865ac71a80619f291eb9ae0761a2cad5df352
4 files changed