[droid] set target SDK to 21and add Android 5.0 use-feature leanback

Not sure if this works as documents are quite unclear about this.
Hopefully some one can actually test this on a ADT-1 (compiled fine using jenkins so doesn't break building).
Could set up a testgroup and upload a alpha version to see if it actually shows up.
@anssih @koying (maybe @Chainfire)