There are Android wallets that have "buy" features built in... but a lot of them include massive mark-ups/commissions... Mycelium for instance uses "Simplex" and allows you to buy with a credit card, but has something like a 10% fee!
Also, it is BTC only... no ETH or LTC support.
Coinomi has support for multiple coins... and conversion between coins with ShapeShift/Changelly etc, but I'm not sure it has any facilities for actually buying coins.
You're possibly better off just using a "traditional" exchange like Coinbase etc to purchase coins, and transfer them to your mobile wallet as required.