Clean MTE and GWP-ASan runtime flags after use.

A check in ZygoteHooks_nativePostForkChild expects runtime_flags to be
empty. Clear the flags that have been used and are not needed any
longer.

Bug: 157939523
Test: adb logcat | grep "Unknown bits set in runtime_flags"
Change-Id: Ifade5e2437cbf24ae1d3909032a98bd60fc8a148