[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 and other devices (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)

Edit:
triggered jenkins builds for ARM/x86 and they will show up in test-builds on the server