[android] remove android:allowBackup from AndroidManifest.xml

[rt2zz]

I am seeing this error on my build when setting backup to false:

* What went wrong:
Execution failed for task ':app:processReleaseManifest'.
> Manifest merger failed : Attribute application@allowBackup value=(false) from AndroidManifest.xml:21:5-32
  	is also present at [me.relex:photodraweeview:1.0.0] AndroidManifest.xml:10:9-35 value=(true).
  	Suggestion: add 'tools:replace="android:allowBackup"' to <application> element at AndroidManifest.xml:19:3-136:17 to override.

[rt2zz]

thoughts @ongakuer ?