Bump targetSdkVersion to "30"

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

       versionCode: 28 -> 30
  targetSdkVersion: 28 -> 30
     minSdkVersion: 21

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

Fix: 189558760
Test: manually verified that LatinIME still is functional
Change-Id: I4bf7588c62fb77bf78d4afcb665e9bfbbef53966