Bump targetSdkVersion to "28" and minSdkVersion to "21"

With this CL, targetSdkVersion and minSdkVersion will be bumped as
follows:

  targetSdkVersion: 23 -> 28
     minSdkVersion: 14 -> 21

As far as we know, there should be no user-visible behavior change on
supported OS versions.

Fix: 119489995
Test: manually verified that LatinIME still is functional
Change-Id: I68f1545778b0c8b44c6850f22f3f5ca25bdb6de0