Implement Play Billing for Trusted Web Activities

[PEConn]

This bug tracks the remaining work for adding Play Billing support to TWAs.

Work:

• [ ] Remove the currency symbol from the value field.
• [ ] Update the Play Billing library.
• [ ] Add more verbose logging.
• [x] Correct PaymentActivity#METHOD_NAME. (PR #178)
• [x] Add the Proxy metadata. (PR #178)
• [x] Decide whether payment-provider has any value given we have twa-play-billing. (PR #177)
• [x] Delete the bundles package. (PR #176)