Android: Speed up project opening by caching the sdk version

The slowness is due to AndroidToolChain::addToEnvironment() being called
a lot on project opening. Thus we need to avoid any file opening or
process on loading.

This patch does 2 things:
a) It removes the call to sdkTargets() which is unneeded. That actually
checks which avds are available.

b) It caches the target sdk inside a named target setting. That means,
we assume that the file does not change behind our backs.

Task-number: QTCREATORBUG-9164

Change-Id: I9287934a0624db0b3e8318b8cf76e605418850c2
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>