Convert Android.mk to Android.bp

1. Converted the main content of the shadertoy_android/Android.mk to a .bp one;
2. The remaining .mk file can be removed after all the .mk files in the
subdirectories are converted.

Bug: 194096505
Test: compared the two apk files built by Make and Soong
Test: TreeHugger
Change-Id: I2e69dd34f39addb352bf9c107f301c8628d51384