See go/jetpack-test-android-migration

Test: make checkbuild
Change-Id: I63edeced1465725cd8d6467cd75ea1acc2608932

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

make build uses 'shared' cert in build/make/target/product/security/.
If LatinIME APK built with AndroidStudio doesn't use the same signing
cert, then you cannot install the APK into AOSP image, which is not
convenient.

With this CL, AndroidStudio build also uses the same signing cert.

Note that shared.keystore was built as follows.

 $ openssl pkcs8 -inform DER -nocrypt                      \
   -in build/make/target/product/security/shared.pk8       \
   -out shared.pem
 $ openssl pkcs12 -export                                  \
   -in build/make/target/product/security/shared.x509.pem  \
   -inkey shared.pem -out shared.p12                       \
   -password pass:android -name AndroidDebugKey
 $ keytool -importkeystore -deststorepass android          \
   -destkeystore shared.keystore -srckeystore shared.p12   \
   -srcstoretype PKCS12 -srcstorepass android

Fix: 110741422
Test: made sure that the APK built with Android Studio can be
      installed to an AOSP build
Change-Id: I471a87e190b15ff02bb06849e4660ed7898897a5