Rewrite for Material Design and KeyStore

This commit contains a long-awaited rewrite to address many long-standing
issues. This patch contains:

1. a totally new UI focused on Material Design compliance
2. all tokens are stored in the Android KeyStore
3. support for sending codes over Bluetooth LE to computers via Jelling
4. extensive unit testing
5. expanded branding options (cf. image= and color=)
6. default branding for major issuers (needs docs)
7. sane behavior surrounding the addition of multiple tokens
8. warnings on weak cryptographic parameters
9. token locking (authentication required to get token code)

All this goodness comes with some costs:

1. Android API 23+ is required. This is necessary for KeyStore access.
2. Manual token adding is gone. Use QR code or otpauth:// link.
3. Use of the KeyStore makes token secrets impossible to extract.
4. Tokens are immutable. Existing changes will be discarded on upgrade.

Closes: freeotp#190
Closes: freeotp#96
Closes: freeotp#78
Closes: freeotp#77
Closes: freeotp#68
Closes: freeotp#55
Closes: freeotp#48
Closes: freeotp#45
Closes: freeotp#43
Closes: freeotp#16
Fixes: freeotp#187
Fixes: freeotp#185
Fixes: freeotp#176
Fixes: freeotp#175
Fixes: freeotp#172
Fixes: freeotp#170
Fixes: freeotp#167
Fixes: freeotp#161
Fixes: freeotp#144
Fixes: freeotp#142
Fixes: freeotp#136
Fixes: freeotp#123
Fixes: freeotp#122
Fixes: freeotp#110
Fixes: freeotp#90
Fixes: freeotp#89
Fixes: freeotp#84
Fixes: freeotp#74
Fixes: freeotp#69
Fixes: freeotp#67
Fixes: freeotp#66
Fixes: freeotp#65
Fixes: freeotp#63
Fixes: freeotp#62
Fixes: freeotp#61
Fixes: freeotp#58
Fixes: freeotp#54
Fixes: freeotp#53
Fixes: freeotp#49
Fixes: freeotp#40
Fixes: freeotp#21
Fixes: freeotp#14
Fixes: freeotp#8
Fixes: freeotp#7
Fixes: freeotp#6

.travis.yml

@@ -4,5 +4,5 @@ android:
   components:
     - tools
     - platform-tools
-    - build-tools-26.0.2
-    - android-26
+    - build-tools-27.0.3
+    - android-27