Tests: Fix unit tests so all tests pass

[robertwu1]

Fixes #1501
Fixes #933
Fixes #1103
Fixes #772
Fixes #346
Fixes #232

In the last couple of Android releases, file permissions have become very tight. Before, we were moving the binary to a specific location, giving execute privileges to it, and running it directly in Java. This method is clearly risky and is blocked.

In order to upgrade the UnitTestRunner app, the following was done.

1. In run_tests.sh, the test binary is put under ${TEST_RUNNER_DIR}/app/src/main/jniLibs.
2. For Android O and over, building AndroidTestRunner moves this binary from ${TEST_RUNNER_DIR}/app/src/main/jniLibs and moves it to /data/app/~~WaKVjcw3K51emFW_rzq8ng==/com.google.oboe.tests.unittestrunner-0A3aNOPlAJ-9TpG86JRNfQ==/lib/arm64/libtestOboe.so
3. For Android O and over, UnitTestRunner now can execute the binary from the new spot (getApplicationInfo().nativeLibraryDir).

The old method is still used for Android N and below.

A couple of other quality of life changes were added in this PR:

1. Cleaned up warnings in MainActivity.java. This adds some StringBuilder and some lambdas.
2. Only print logs in ${TEST_RUNNER_PACKAGE_NAME}.MainActivity and OboeAudio instead of all logcats at the end of ./run_tests.sh.
3. Removed empty assets folder in favor of creating jniLibs and assets dynamically. This is the .gitignore change.
4. Clean up build files on every build. When testing on different devices, old build devices makes the compile fail.
5. Loosen requirements on tests so emulators can pass.

All the Android emulators and real phones I tested with pass with the changes.

One of the test failures is actually a code failure in OpenSL ES. I have opened #1582 to fix this.

[philburk]

Great work!