Add BLW

This commit adds a BLW app (Bitcoin Lightning Wallet).

A note on how building is done: prepare_fdroid script (https://github.com/btcontract/lnwallet/blob/master/app/prepare_fdroid.sh) fetches binaries required by app from https://github.com/ElementsProject/libwally-core/releases/tag/release_0.6.3 and verifies their checksum, is this okay?

UPDATE:

• NDK is used now to build the binaries from sources.
• Also added fastlane data to target repo.

• The app complies with the inclusion criteria
• The original app author has been notified (and supports the inclusion)
• All related fdroiddata and RFP issues have been referenced in this merge request
• The upstream app source code repo contains the app metadata (summary/description/images/changelog/etc) in a Fastlane or Triple-T folder structure
• Builds with fdroid build
• Releases are tagged

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