Use Android functionality with minSdkVersion of 14

[str4d]

The Android support libraries, as of version 28 IIRC, now all require minSdkVersion = 14. This would finally drop support for some very old Android versions that we have maintained support for, but given that the I2P Android app has already been bumped to that minimum, and given the almost negligible user base of those older versions, there's no significant reason to not do so here as well.

Things to do:

• Look for any backwards-incompatible changes in newer support libraries.
• Check whether there is anything we can drop or improve after making the changes.
• Update any Android libraries we depend on that were blocking on us updating minSdkVersion.
• Update the various underlying Android libraries that we ourselves developed, to also require minSdkVersion = 14.

[str4d]

#118 bumped minSdkVersion, so this issue now just covers the various changes we can make now that we don't have to support pre-14 API versions.