See https://developer.android.com/studio/releases/gradle-plugin to check
relationship between AGP and gradle version. Current project uses gradle
6.8.3, so this CL uses command `gradle wrapper --gradle-version 6.8.3`
to generate gradle wrapper.
This CL also remove ignore from .gitignore for gradle wrapper files. It
is very useful to build monado Android version with command:
`./gradlew :src:xrt:targets:openxr_android:build`
Signed-off-by: utzcoz <utzcoz@outlook.com>