Add workaround to support Samsung DeX

[chenxiaolong]

This commit changes the Magikeyboard service behavior so that KeePassDX
is able to run in Samsung DeX mode. Currently, the app cannot run in
DeX mode because apps which have services using BIND_INPUT_METHOD are
blocked.

A new broadcast receiver has been added to listen for DeX's enter/leave
events [1] and disable/enable the Magikeyboard service appropriately.
The enabled state of a service lives in the Android framework's
PackageManager and survives app crashes and device reboots (though it
does get reset when app data is cleared).

Additionally, an extra check is added to FileDatabaseSelectActivity to
ensure the service's enabled state is correct. This is necessary if the
app crashes or is force quit within DeX mode and then the user exits DeX
mode. Otherwise, the service would stay disabled until the user entered
and exited DeX again.

With the new behavior, KeePassDX will generally just work with DeX,
though there's one caveat: after the initial installation, the user must
open the app once outside of DeX. Otherwise, Android will not trigger
the broadcast receiver. This could be fixed by making the service
intially disabled in the manifest with android:enabled="false", but
Android's Settings app in SDK 15 through 25 does not correctly refresh
the keyboard list when changing the service from disabled to enabled.
I opted not to introduce different behavior based on the API version.

[1] https://developer.samsung.com/sdp/blog/en-us/2017/07/27/samsung-dex-how-to-detect-the-samsung-dex-mode

Fixes: #245

[chenxiaolong]

I've tested this in the Android emulator for every SDK version from 15 through 31 and also on my Samsung tablet (inside and outside DeX mode). I don't have non-Samsung real devices to test with.

(Also, please feel free to close this without merging if it is too hacky 🙂)

[J-Jamet]

I think this is a good approach. I'll test on my devices to see if I don't get any side effects for normal deployment, and I trust you on how the DEX works. ;)