Fix targetSdkVersion for NetworkStack

targetSdkVersion was set to 28 in aosp/916713, however this was broken
by the manifest refactor in aosp/923338: the targetSdk is not preserved
in the manifest merge.

Test: aapt dump badging shows targetSdkVersion 28
Change-Id: I7d06b141856e96ef011db32094bfc62a8cd1b350
3 files changed