Workarounds to make the RTDB testapp work with latest Android Studio.

[var-const]

Using Android Studio 3.4.2 and Pixel 2 XL (Android 9, API 28) emulator, trying to compile the testapp produces the following errors:

• ERROR: The minSdk version should not be declared in the android manifest file. You can move the version from the manifest to the defaultConfig in the build.gradle file. Solution/workaround: just remove the element from the XML file;
• error: resource android:attr/fontVariationSettings not found and error: resource android:attr/ttcIndex not found. Solution/workaround: increase compileSdkVersion in build.gradle to 28 (taken from here: https://stackoverflow.com/questions/51291407/androidx-modules-androidattr-ttcindex-androidattr-fontvariationsettings-not/51291698);
• many errors like:

  libfirebase_database.a(data_snapshot.o): relocation R_386_GOTOFF
  against preemptible symbol
  _ZN8firebase8database9CleanupFnINS0_12DataSnapshotENS0_8internal20DataSnapshotInternalEE21create_invalid_objectE
  cannot be used when making a shared object

  Solution/workaround: add the following to CMakeLists.txt:

  set(CMAKE_SHARED_LINKER_FLAGS
  "${CMAKE_SHARED_LINKER_FLAGS}
  -Wl,--exclude-libs,libfirebase_database.a
  -Wl,--exclude-libs,libfirebase_auth.a
  -Wl,--exclude-libs,libfirebase_app.a")

  (taken from here: https://github.com/opencv/opencv/issues/10229#issuecomment-359202825)