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:
opencv/opencv#10229 (comment))