libbinder_ndk: ScopedAStatus: remaining helpers

This allows, say:
return ScopedAStatus(AStatus_fromExceptionCode(EX_UNSUPPORTED_OPERATION));
=>
return ScopedAStatus::fromExceptionCode(EX_UNSUPPORTED_OPERATION);

It only saves a couple of characters, but not having to type 'AStatus'
twice is a huge psychological win.

Bug: 136027762
Test: using these
Change-Id: Idc574fa1377c98f382cd985e39ab20488853ecf8